biol. rev. (2010), 85, pp. 55–110. doi:10.1111/j.1469-185x

Biol. Rev. (2010), 85, pp. 55–110. 55doi:10.1111/j.1469-185X.2009.00094.x

The origin and early evolution of dinosaurs

Max C. Langer1∗, Martin D. Ezcurra2, Jonathas S. Bittencourt1

and Fernando E. Novas2,3

1Departamento de Biologia, FFCLRP, Universidade de Sao Paulo; Av. Bandeirantes 3900, Ribeirao Preto-SP, Brazil2Laboratorio de Anatomia Comparada y Evolucion de los Vertebrados, Museo Argentino de Ciencias Naturales ‘‘Bernardino Rivadavia’’, Avda.

Angel Gallardo 470, Cdad. de Buenos Aires, Argentina3CONICET (Consejo Nacional de Investigaciones Cientıficas y Tecnicas); Avda. Rivadavia 1917 - Cdad. de Buenos Aires, Argentina

(Received 28 November 2008; revised 09 July 2009; accepted 14 July 2009)

ABSTRACT

The oldest unequivocal records of Dinosauria were unearthed from Late Triassic rocks (approximately 230 Ma)accumulated over extensional rift basins in southwestern Pangea. The better known of these are Herrerasaurus

ischigualastensis, Pisanosaurus mertii, Eoraptor lunensis, and Panphagia protos from the Ischigualasto Formation, Argentina,and Staurikosaurus pricei and Saturnalia tupiniquim from the Santa Maria Formation, Brazil. No uncontroversialdinosaur body fossils are known from older strata, but the Middle Triassic origin of the lineage may be inferredfrom both the footprint record and its sister-group relation to Ladinian basal dinosauromorphs. These includethe typical Marasuchus lilloensis, more basal forms such as Lagerpeton and Dromomeron, as well as silesaurids: a possiblymonophyletic group composed of Mid-Late Triassic forms that may represent immediate sister taxa to dinosaurs.The first phylogenetic definition to fit the current understanding of Dinosauria as a node-based taxon solelycomposed of mutually exclusive Saurischia and Ornithischia was given as ‘‘all descendants of the most recentcommon ancestor of birds and Triceratops’’. Recent cladistic analyses of early dinosaurs agree that Pisanosaurus mertii

is a basal ornithischian; that Herrerasaurus ischigualastensis and Staurikosaurus pricei belong in a monophyletic Her-rerasauridae; that herrerasaurids, Eoraptor lunensis, and Guaibasaurus candelariensis are saurischians; that Saurischiaincludes two main groups, Sauropodomorpha and Theropoda; and that Saturnalia tupiniquim is a basal member ofthe sauropodomorph lineage. On the contrary, several aspects of basal dinosaur phylogeny remain controversial,including the position of herrerasaurids, E. lunensis, and G. candelariensis as basal theropods or basal saurischians,and the affinity and/or validity of more fragmentary taxa such as Agnosphitys cromhallensis, Alwalkeria maleriensis,Chindesaurus bryansmalli, Saltopus elginensis, and Spondylosoma absconditum. The identification of dinosaur apomorphiesis jeopardized by the incompleteness of skeletal remains attributed to most basal dinosauromorphs, the skulls andforelimbs of which are particularly poorly known. Nonetheless, Dinosauria can be diagnosed by a suite of derivedtraits, most of which are related to the anatomy of the pelvic girdle and limb. Some of these are connected to theacquisition of a fully erect bipedal gait, which has been traditionally suggested to represent a key adaptation thatallowed, or even promoted, dinosaur radiation during Late Triassic times. Yet, contrary to the classical ‘‘compet-itive’’ models, dinosaurs did not gradually replace other terrestrial tetrapods over the Late Triassic. In fact, theradiation of the group comprises at least three landmark moments, separated by controversial (Carnian-Norian,Triassic-Jurassic) extinction events. These are mainly characterized by early diversification in Carnian times, aNorian increase in diversity and (especially) abundance, and the occupation of new niches from the Early Jurassiconwards. Dinosaurs arose from fully bipedal ancestors, the diet of which may have been carnivorous or omnivo-rous. Whereas the oldest dinosaurs were geographically restricted to south Pangea, including rare ornithischiansand more abundant basal members of the saurischian lineage, the group achieved a nearly global distribution bythe latest Triassic, especially with the radiation of saurischian groups such as ‘‘prosauropods’’ and coelophysoids.

Key words: Dinosauria, Dinosauromorpha, Triassic, phylogeny, evolution, biogeography, Herrerasauria.

∗ Address for correspondence: Departamento de Biologia, Faculdade de Filosofia, Ciencias e Letras de Ribeirao Preto, Universidadede Sao Paulo; Av. Bandeirantes 3900, 14040-901, Ribeirao Preto-SP, Brazil (Tel: +55 16 3602 3844; Fax: +55 16 3602 4886;E-mail: [email protected])

Biological Reviews 85 (2010) 55–110 © 2009 The Authors. Journal compilation © 2009 Cambridge Philosophical Society

56 Max C. Langer and others

CONTENTS

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56(1) Historical background on early dinosaurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56(2) The dinosauromorph radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

II. Phylogeny and Systematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59(1) What makes a dinosaur? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60(2) Phylogenetic definitions: naming early dinosaurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

III. Dinosaur ‘‘Trail Blazers’’ in Space, Time, and Evolutionary Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67(1) The oldest dinosaurs and the rocks that contain them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67(2) The evolutionary tree of early dinosaurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71(3) Geographical distribution of basal dinosaurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

IV. Ecology of the Dinosaur Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77(1) The Triassic scene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77(2) Lucky break? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78(3) Of legs and teeth: insights on the palaeobiology of early dinosaurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

V. Outcomes of a Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84(1) Early ornithischian evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84(2) Early sauropodomorph evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87(3) Early theropod evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93VII. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

VIII. Appendix 1. Institutional Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94IX. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

I. INTRODUCTION

Dinosaurs originated in the Triassic period, and theLate Triassic represents the first 30 of the 165 millionyears of their ‘‘non-avian’’ history on Earth. Yet, ofthe 500-700 ‘‘well established’’ dinosaur genera (Wang& Dodson, 2006; Olshevsky, 2007), only about 30(approximately 5%) were excavated from Triassic rocks,and the diversity/diversification of the group is mainlyconcentrated in the Jurassic (Rauhut, 2005b; Lloyd et al.,2008) and/or Cretaceous (Wang & Dodson, 2006) periods.This is especially the case if one accepts the inference ofWang & Dodson (2006) that the Late Triassic representsthe best sampled subperiod of the entire Mesozoic in termsof documented dinosaur diversity. Indeed, dinosaurs arerare in most Triassic fossil assemblages in which they occur,although by the end of the period they were already dominantmembers of various palaeocommunities.

Triassic dinosaurs were mostly bipedal, and not exception-ally large. The basal-most forms were probably omnivorous,but predatory and probably herbivorous dinosaurs alsooccurred during Late Triassic times. This includes Her-

rerasaurus ischigualastensis, a top predator up to 4 m long(Novas, 1997a), and Riojasaurus incertus, a plant-eater of aboutthree tons (Seebacher, 2001). In taxonomic terms, most Tri-assic dinosaurs are regarded as members of one of the threemajor lineages of the group: Theropoda, Sauropodomorpha,and Ornithischia. Yet, despite representing well-known taxa,other Triassic dinosaurs have a debated phylogenetic posi-tion. This is particularly the case of the herrerasaurs, whichwere placed basal to the Ornithischia-Saurischia dichotomy,

nested within Theropoda, or regarded as non-eusaurischansaurischians.

Appealing inferences on dinosaur palaeobiology canbe drawn from eggs and nestlings (Bonaparte & Vince,1979; Moratalla & Powell, 1994), monospecific assemblages(Coombs, 1990; Schwartz & Gillette, 1994), visual-display-related morphological features (Vickaryous & Ryan, 1997),and ‘‘stomach contents’’ (Novas, 1997a; Nesbitt et al., 2006)of Triassic dinosaurs. Yet, the most debated aspect of earlydinosaur macroevolution corresponds to their first radiation,and various scenarios were invoked to explain the rise ofthe clade in a time interval during which most terrestrialtetrapods suffered important diversity losses. In fact, by LateTriassic times, dinosaurs arose and took their first stepsalong the evolutionary road, and the investigation of theirobscure origins is crucial for the understanding of dinosaurinterrelationships and palaeobiology as a whole.

(1) Historical background on early dinosaurs

Research on early dinosaurs can be said to have startedwith the work of the German palaeontologist Friedrich vonHuene, and his descriptions of Saltopus elginensis Huene,1910 (Fig. 1A), and Spondylosoma absconditum Huene, 1942.These two forms have completely different provenances,coming respectively from the Elgin area, in NorthernScotland, and Rio Grande do Sul, in South Brazil, butshare curious similarities. Both were regarded as saurischiandinosaurs by Huene (1910, 1942) and were found in depositsconsidered the oldest dinosaur-bearing rocks known at thetime. Huene (1932, 1942) identified various other putativeTriassic dinosaurs as equivalent in age to either Saltopus orSpondylosoma, but most of these were shown to have doubtful


The origin and early evolution of dinosaurs 57

dinosaur affinities (Galton, 1985b; Benton, 1986b; Galton &Walker, 1996; Benton et al., 2000; Rauhut & Hungerbuhler,2000; Parker et al., 2005; Nesbitt, Irmis & Parker, 2007).Notable exceptions are Thecodontosaurus antiquus (Benton et al.,2000) and the material Cope (1889) originally assigned toCoelophysis bauri (Nesbitt et al., 2007), but these came fromstrata currently considered younger (Benton et al., 2000;Langer, 2005b; Nesbitt et al., 2007). Indeed, the older age ofboth the ‘‘Stagonolepis-beds’’ of Elgin (Huene, 1908) and the‘‘Rio do Rasto’’ [sic] beds at Chiniqua (Huene & Stahlecker,1931) was corroborated by recent work. The LossiemouthSandstone Formation has been dated as Carnian (Benton &Walker, 1985), whereas the Dinodontosaurus Assemblage-Zoneof the Santa Maria Formation is considered of Ladinianage (Langer et al., 2007c); or early-middle Carnian, followingrecent modifications on the Late Triassic time-scale (Muttoniet al., 2004) and the corrections on the radiometric dating ofthe Ischigualasto Formation (Furin et al., 2006).

Although the ages of the Lossiemouth Sandstone andSanta Maria formations were more securely established,the dinosaur affinities of Saltopus elginensis and Spondylosoma

absconditum are still debated (Rauhut & Hungerbuhler, 2000;Galton, 2000; Langer, 2004). This is in part due to thepoor preservation of the specimens, which do not allow acomprehensive assessment of their morphological features.Therefore, it was not until Reig (1963) placed Herrerasaurus

ischigualastensis (Fig. 1B) and Ischisaurus cattoi within Saurischiathat unequivocal early dinosaurs were known to science. Thedescribed specimens were collected in 1961 from depositsof the Ischigualasto Formation, San Juan province, north-western Argentina, which have yielded remains attributableto dinosaurs since the late 1950s (Reig, 1963). With thediscovery, in 1962, of the ornithischian Pisanosaurus mertii

Casamiquela, 1967 (Fig. 1C), in that same stratigraphic unit,the presence of both main dinosaur lineages (i.e. Ornithischiaand Saurischia), in the Triassic of South America wasconfirmed. Another important ‘‘early dinosaur’’ study ofthe time was the description of Staurikosaurus pricei Colbert,1970 (Fig. 1D). Its type and only specimen, discovered in

1936 in the Santa Maria beds of South Brazil, was the firstconsensual early dinosaur to be collected.

While the 1980s were quiet times regarding the studyof early dinosaurs, mainly witnessing the description ofincomplete specimens (Galton, 1985b, 1986; Novas, 1986;Chatterjee, 1987; Murry & Long, 1989), the early ninetiescame with new and exciting discoveries. These include theunearthing, also from the Ischigualasto Formation, of a newbasal dinosaur still to be fully described, Eoraptor lunensis

(Sereno et al., 1993; Sereno, 2007b), and of further materialof Herrerasaurus ischigualastensis (Sereno & Novas, 1992, 1993;Novas, 1993; Sereno, 1993). In the late nineties, a new seriesof discoveries in Rio Grande do Sul, South Brazil, enlargedthe knowledge of early dinosaur diversity. The then basal-most member of the sauropodomorph lineage, Saturnalia

tupiniquim (Langer et al., 1999; Langer, Franca & Gabriel,2007b; Langer, 2003), was unearthed from the Hyperodapedon

Assemblage-Zone of the Santa Maria Formation, whereasthe overlying Caturrita Formation yielded the saurischianGuaibasaurus candelariensis (Bonaparte, Ferigolo & Ribeiro,1999; Bonaparte et al., 2007). Since the beginning of thiscentury, some putative basal dinosaurs have been described(Fraser et al., 2002; Ferigolo & Langer, 2007; Nesbitt et al.,2007; Nesbitt & Chatterjee, 2008; Martinez & Alcober, 2009;Ezcurra, 2008), while the validity of others was evaluated inthe light of new evidence (Remes & Rauhut, 2005; Yates,2007b). More importantly, different evolutionary scenarioswere proposed based on independent cladistic analyses, e.g.Langer & Benton (2006), Ezcurra (2006), Sereno (2007b),Irmis et al. (2007a), which attempted to sum up informationin order to understand better the interrelationships of earlydinosaurs.

(2) The dinosauromorph radiation

For most of the last century, it was accepted that dinosaursarose from ‘‘thecodont’’ precursors, either as a monophyleticgroup or, more frequently (Fig. 2), in the form of inde-pendent lineages (Huene, 1956; Colbert, 1964; Charig,

Fig. 1. Early images depicting some of the oldest putative dinosaurs. (A) Drawing of the slabs containing Saltopus elginensis, fromHuene (1910). (B) Skeletal reconstruction of Herrerasaurus ischigualastensis as mounted in 1965 for exhibition at the UniversidadNacional de Tucuman, from Bonaparte (1997). (C) Skeletal reconstruction of Pisanosaurus mertii, from Bonaparte (1997). (D) Skeletalreconstruction of Staurikosaurus pricei, from Colbert (1970). Scale bars: A = 5 cm; B-D = 10 cm.



Fig. 2. Schemes of archosaur evolution depicting a polyphyletic Dinosauria. (A) Modified from Krebs (1974). (B) After Thulborn(1975).

Attridge & Crompton, 1965; Romer, 1966). ‘‘Thecodonts’’,as composed of non-crown-group archosaurs, and basalmembers of both the bird and crocodile lines, are cur-rently regarded as a paraphyletic group (Currie & Padian,1997b; Benton, 2004). In his seminal paper on dinosaurphylogeny, Gauthier (1986) applied the name OrnithosuchiaHuene, 1908, to designate a group composed of dinosaurs,pterosaurs (including Scleromochlus), ornithosuchids, Euparke-

ria (questionably), and ‘‘Lagosuchus’’, a small archosaur fromthe Middle Triassic of Argentina (Romer, 1971; Bonaparte,1975; Sereno & Arcucci, 1994). That clade was supposed togroup all archosaurs that share a closer affinity to birds (withinDinosauria) than to crocodiles, which were placed in its sis-ter group Pseudosuchia (Parrish, 1997; Senter, 2005). Morerecent work, however, excluded both Euparkeria (Benton &Clark, 1988; Sereno, 1991a; Juul, 1994; Benton, 2004) andornithosuchids (Sereno, 1991a; Juul, 1994; Benton, 2004)from Ornithosuchia, restricting the inclusivity, and perhapsworthiness (Taylor, 2007) of the name. Indeed, alternativenames were later proposed for the bird line of Archosauria,e.g. Avemetatarsalia Benton, 1999; Panaves Gauthier & DeQueiroz, 2001. The inclusivity of this group could be evenmore reduced considering the labile position of pterosaurs,sometimes regarded as basal archosaurs (Bennett, 1996) oreven outside Archosauria (Peters, 2000; Sobral & Langer,2008). In this scenario, the non-dinosaur members of thebird-lineage of Archosauria would only include Scleromochlus

taylori (a putative sister taxon to Pterosauria) from the LateTriassic of Elgin (Sereno, 1991a; Benton, 1999) and theso-called basal dinosauromorphs.

The name Dinosauromorpha was coined by Benton(1985) to include dinosaurs, birds, and ornithosuchids, butredefined by Sereno (1991a) to its current understanding,which excludes ornithosuchids. The basal (non-dinosaurian)

members of the group (Romer, 1971, 1972a, b; Arcucci,1987) were for a long time known only from the MiddleTriassic Chanares Formation of Argentina (Rogers et al.,2001). These small, gracile forms were grouped within‘‘Pseudosuchia’’, but were soon recognized to have somebearing on the origin of dinosaurs (Romer, 1972a, b), whichbecame evident with the works of Bonaparte (1975), Sereno& Arcucci (1993, 1994), and Novas (1996). Those authorsidentified typical dinosaur hind-limb traits on these taxa,including a distally tapering fibula, an anterior ascendingprocess in the astragalus, a reduced calcaneum, a longermetatarsus with reduced outer elements, and a straightmetatarsal V with reduced articulation area on the outersurface of the lateral distal tarsal (see also Irmis et al.,2007a; Brusatte et al., 2009). The taxonomy of the Chanaresdinosauromorphs has always been subject to some debate(Bonaparte, 1975, 1995; Sereno & Arcucci, 1994; Arcucci,1987, 1998, 2005), and five names entered the literature:Lagerpeton chanarensis Romer, 1971; Lagosuchus talampayensis

Romer, 1971 (nomen dubium; Sereno & Arcucci, 1994);Marasuchus lilloensis (Romer, 1972b, gen. Sereno & Arcucci,1994); Lewisuchus admixtus Romer, 1972a (Arcucci, 1997); andPseudolagosuchus major Arcucci, 1987.

Recent discoveries (Fraser et al., 2002; Dzik, 2003; Irmiset al., 2007a; Ferigolo & Langer, 2007) and interpretations(Novas & Ezcurra, 2005; Ezcurra, 2006; Nesbitt et al., 2007)suggest that basal dinosauromorphs were both more diversein terms of anatomy and inferred habits, and more widelyspread chronologically and geographically. Dromomeron romeri

and D. gregorii (Irmis et al., 2007a; Nesbitt et al., 2009) wererecognized in the Norian of western North America, whichalso yielded Eucoelophysis baldwini. The latter taxon, firstdescribed as a theropod dinosaur (Sullivan & Lucas, 1999),was reassigned to a non-dinosaur dinosauriform position,



as either the sister taxon to Dinosauria (Ezcurra, 2006) orforming a group with Silesaurus opolensis (Irmis et al., 2007a).The latter form, collected in Carnian deposits of Poland(Dzik, 2003; Dzik & Sulej, 2007), provided the greatestbreakthrough in the recent study of dinosaur origins. Its longfore limbs suggest that the animal was at least facultativelyquadrupedal, while the edentulous front tip of its lower jawapparently bore a corneous beak. This atypical set of traitsrevealed an unsuspected morphological diversity, hinting athow incomplete was, and certainly still is, our knowledge ofthe early stages of dinosauromorph evolution. In addition,the record of Silesaurus opolensis extended the range of basaldinosauriforms into the Late Triassic of Europe, a possibilityonly hinted at before on the basis of controversial British taxasuch as Saltopus elginensis (Rauhut & Hungerbuhler, 2000) andAgnosphitys cromhallensis (Fraser et al., 2002). Further, sincethe description of Silesaurus opolensis, newly and previouslydescribed Norian forms have been considered closely relatedto the taxon. This is the case for Sacisaurus agudoensis Ferigolo& Langer, 2007, from the Caturrita Formation of SouthBrazil, and a set of North American specimens (Nesbittet al., 2007), including material assigned to an unnamedSilesaurus-like form from the Petrified Forest Member, ChinleFormation, of New Mexico, and part of the original materialof Technosaurus smalli Chatterjee, 1984, from the Bull CanyonFormation, Texas (Irmis et al., 2007b). The latter taxonhas been previously assigned to Ornithischia (Weishampel& Witmer, 1990; Sereno, 1991b; Hunt & Lucas, 1994),while Sacisaurus agudoensis might provide evidence that evenSilesaurus opolensis represents a basal member of that dinosaurclade (Ferigolo & Langer, 2007).

The more complete non-dinosaurian dinosaurormorphsform a series of outgroups to Dinosauria, and they giveclues about the origin of the clade (Ezcurra, 2006; Langer &Benton, 2006; Yates, 2007a; Irmis et al., 2007a; Brusatteet al., 2009). The long-held hypothesis of a more basalposition for Lagerpeton chanarensis (Novas, 1992b; Sereno& Arcucci, 1993) was confirmed by independent studies(Irmis et al., 2007a; Brusatte et al., 2009), which allocatedthe genus Dromomeron as its sister taxon (Fig. 3A). BothLagerpeton and Dromomeron lack several apomorphic featuresof Dinosauriformes such as a reduced medial lamina on thepubis, an antitrochanter expanding into the ilium, a lessertrochanter on the proximal femur, and a distal tibia bearinga lateral groove and a squared distal articulation (Irmis et al.,2007a; Brusatte et al., 2009). Within Dinosauriformes, moststudies (Novas, 1992b, 1996; Ezcurra, 2006; Irmis et al.,2007a; Brusatte et al., 2009) place Marasuchus lilloensis as thebasalmost member of the clade (Fig. 3A). More derived formsinclude Pseudolagosuchus major (Novas, 1992b, 1996) and itspossible senior synonym Lewisuchus admixtus (Arcucci, 1998,2005). Along with the identification of further dinosauriformsof equivalent grade (Dzik, 2003; Ezcurra, 2006), twoalternative phylogenetic scenarios were proposed (Fig. 3A).Irmis et al. (2007a) suggested that Eucoelophysis and Silesaurus

form the sister clade to Dinosauria, which may also includePseudolagosuchus according to Nesbitt et al. (2007, p. 214).

Ezcurra (2006), on the other hand, placed all these taxa ina fully pectinated grade, where Pseudolagosuchus, Silesaurus,and Eucoelophysis, are respectively closer to Dinosauria.A somewhat intermediate view was adopted by Brusatteet al. (2009), in which Pseudolagosuchus has a basal position,and Lewisuchus forms, with other taxa, a more restricted sisterclade to dinosaurs (Fig. 3A). In any case, all or some of theseforms share with dinosaurs a number of apomorphic traitsabsent in Marasuchus, e.g. longer pubic shaft; femur withangular greater trochanter, ‘‘spike-like’’ lesser trochanter,and prominent trochanteric shelf; distal tibia with laterallyexpanded outer malleolus; astragalus with pyramid-shapedanterior ascending process; and sigmoidal metatarsal IVwith deeper distal articular surface (Novas, 1996; Irmis et al.,2007a; Brusatte et al., 2009).

Regardless of their status as a clade or ‘‘grade’’, these morederived basal dinosauromorphs fill a gap (between Marasuchus

lilloensis and dinosaurs) in archosaur evolution. Moreimportantly, they fill that gap with the unsuspected diversityof forms that have been informally called ‘‘silesaurids’’.This group may just include Silesaurus, and forms such asSacisaurus and Technosaurus, which share with the Polish taxondental/jaw features possibly related to a more herbivorousdiet (Ferigolo & Langer, 2007; Irmis et al., 2007b), but it couldalso encompass Lewisuchus, Pseudolagosuchus, and Eucoelophysis.Although the basis for this assignment lies on shared traitsof the postcranium, there is no positive evidence that anyof these forms was a facultative/full quadruped as Silesaurus.Yet, ‘‘herbivorous’’ teeth have been tentatively referred toEucoelophysis (Irmis et al., 2007a). The record of ‘‘silesaurids’’and of the species of Dromomeron suggests that an extensiveradiation of non-dinosaurian dinosauromorphs preceded theLate Triassic dinosaur diversification, and that parallel to thefirst radiation of dinosaurs, that grade continued to flourishafter the Ladinian (Irmis et al., 2007a), extending their rangeinto the northern part of west Pangea (Fig. 3B).

II. PHYLOGENY AND SYSTEMATICS

The name Dinosauria was erected by Owen (1842) toinclude three large terrestrial forms which he believed tocompose a distinct group of extinct reptiles (Torrens, 1992;Padian, 1997a). In the following years, a sound conceptof Dinosauria was established by the proposition of severalclassification schemes (Cope, 1866; Huxley, 1870; Marsh,1882; Seeley, 1888). At that time, major taxa such asSauropoda and Theropoda (Marsh, 1878, 1881), as well asSaurischia and Ornithischia (Seeley, 1888) were proposed.These names gained acceptance in the 20th Century (Huene,1932; Romer, 1956) and still represent the major dinosaursubdivisions as currently understood (Fig. 4). However, formost of the last century these different dinosaur groups, andeven some of their subgroups, were believed to have hadindependent origins (Fig. 2) from ‘‘thecodont’’ precursors(Huene, 1914, 1956; Colbert, 1964; Charig et al., 1965;Romer, 1966; Reig, 1970; Krebs, 1974; Thulborn, 1975;



Fig. 3. Time-calibrated phylogenies and distribution of non-dinosaur Dinosauromorpha. (A) Recently proposed phylogenetichypotheses; dotted lines indicate ghost lineages; names applied as in Table 1. Position of Pseudolagosuchus in the phylogeny of Irmiset al. (2007a) inferred from Nesbitt et al. (2007). (B) Geographic occurrence of taxa on a Late Triassic map redrawn from Blakey(2006). Black silhouettes adapted from various sources.

Cruickshank, 1975, 1979). The monophyly of Dinosauria wassuggested by Bakker & Galton (1974) and Bonaparte (1975,1976), firmly established by various pioneering cladisticworks (Paul, 1984; Gauthier & Padian, 1985; Cooper, 1985;Brinkman & Sues, 1987), especially that of Gauthier (1986),and represents a consensual hypothesis nowadays (Novas,1989; 1996; Sereno et al., 1993; Sereno, 1999; Langer &Benton, 2006; Irmis et al., 2007a).

(1) What makes a dinosaur?

Even if the monophyly of Dinosauria is consensuallyaccepted, the issue of which morphological traits characterizethe group continues to be debated (Novas, 1996; Langer &Benton, 2006; Sereno, 2007b). Several putative dinosaur

apomorphies were proposed in a variety of studies dealingwith the phylogeny of the group, which frequently divergeupon the distribution of these same characters. This isepitomized by the continuing quarrel over one of thediagnostic features mentioned by Owen (1842) in the originalproposition of the name: the number of vertebrae thatcompose the dinosaur sacrum. In the following text, mostrecent reviews of early dinosaur phylogeny (Novas, 1996;Sereno, 1999, 2007a; Fraser et al., 2002; Benton, 2004;Langer & Benton, 2006; Ezcurra, 2006; Yates, 2007a, b;Irmis et al., 2007a; Brusatte et al., 2008a) are compared andevaluated, in a search for the set of traits that typicallycharacterize the group. Obviously, a key point to set thediagnosis of Dinosauria is to determine whether some ofthe so-called basal dinosauromorphs actually belong to the



Fig. 4. Generalized phylogeny depicting the position of Dinosauria and its main groups within Archosauria. Dotted lines indicatemajor contentious placement of taxa; arrows indicate stem-based taxa; black circles indicate node-based taxa; names applied as inTable 1; black silhouettes adapted from various sources.

group. As reviewed by Langer & Benton (2006, pp. 316-317),various putative dinosaur apomorphies are seen in Silesaurus

opolensis. These might represent true dinosaur apomorphiesif the taxon is considered to represent a basal ornithischian(Ferigolo & Langer, 2007). Yet, current orthodoxy pointstowards the basal, non-dinosaurian position of Silesaurus, andthis hypothesis of relationships represents the template basedon which the unique dinosaur traits are discussed below.

Novas (1996) and Sereno (1999) respectively listed 17 and18 characters as diagnostic for Dinosauria, while a modifiedversion of one of their characters (presence of three or moresacral vertebrae) is the sole dinosaur apomorphy proposedby Fraser et al. (2002). Langer & Benton (2006) criticallyassessed these characters, questioning the apomorphic statusof several of them. Features related to the cranial anatomy(Sereno & Novas, 1993) are particularly problematic becausemost basal dinosaurs and, especially, basal dinosauromorphslack good skull material. Indeed, traits such as the lack of thepostfrontal bone, although typically absent in non-dinosaurarchosaurs and present in dinosaurs (see Irmis et al., 2007a,char. 14), can not be considered an unambiguous dinosaurapomorphy (Langer & Benton, 2006) given its equivocaloccurrence in most forms placed at the very origin of thegroup. The same applies to other putative apomorphies ofthe dinosaur skull, such as the dorsal overlap of the transverseflange of the pterygoid by the ectopterygoid, and the lateralexposure of the quadrate head (Langer & Benton, 2006); seealso Brusatte et al. (2008a, chars 10, 14, 38, 40, 67). Thestatus of other putative apomorphies of the dinosaur skull isdependent on the position of Silesaurus opolensis, the cranialmaterial of which is reasonably complete (Dzik, 2003; Dzik& Sulej, 2007). If not considered a dinosaur, some of its

cranial traits, e.g. frontal participating in the supratemporalfossa, are dismissed as dinosaur apomorphies. Yet, if itsless consensual position as a basal ornithischian is accepted,these same traits continue potentially to represent dinosaursynapomorphies. On the contrary, plesiomorphic traits inthe skull of Silesaurus such as a large post-temporal fenestrasupports its non-dinosaurian affinity, and helps to define areduced foramen-sized aperture (Fig. 5B) as apomorphic fordinosaurs (Irmis et al., 2007a, char. 21). Other cranial features(Langer & Benton, 2006, char. 12; Ezcurra, 2006, chars 4,20; Yates, 2007a, chars 26, 29; Irmis et al., 2007a, chars 2,25) suggested to represent possible dinosaur apomorphies,pending the criteria used for character optimization, have anerratic distribution among basal dinosaurs, and should notbe considered a priori diagnostic traits of the group. A likelydinosaur apomorphy, related to the axial skeleton (Fig. 5C), isthe presence of epipophyses on the cranial cervical vertebrae(Novas, 1996; Langer & Benton, 2006; Yates, 2007a; contra

Ezcurra, 2006). This feature was previously considered asaurischian apomorphy, but more recently was recorded inbasal ornithischians (Novas, 1996; Langer & Benton, 2006;Butler, Smith & Norman, 2007). Other putative apomorphiesof the dinosaur vertebral column listed by Yates (2007a, chars129, 142) have an inconsistent distribution, and should notbe a priori considered as such.

As mentioned earlier, the increase in the number ofvertebrae that forms the dinosaur sacrum (from two tomore than two) continues to be listed as an apomorphy ofthe group (Novas, 1996; Sereno, 1999; Fraser et al., 2002;Ezcurra, 2006). Recently, as discussed by Langer & Benton(2006), two main strategies of coding characters related tothis transformation have been employed; but see Novas



Fig. 5. The dinosaur Plateosaurus engelhardti. (A) Skeletal reconstruction (from Yates, 2003a), with indications of the better knownapomorphic traits of Dinosauria. (B) Occipital view of the skull (from Galton, 1985a) indicating (1) a foramen-sized post-temporalfenestra. (C) Lateral view of a cervical vertebra, indicating (2) the presence of epipophyses. (D) Caudal view of the left humerus,indicating (3) a long deltopectoral crest. (E) Lateral view of the left ilium, indicating (4) an open acetabulum and (5) an arched dorsalmargin. (F) Cranial view of the left femur, indicating (6) a femoral head inturned and distinctly offset from the shaft and (7) anasymmetrical forth trochanter. (G) Proximal view of the left astragalus, indicating (8) an acute anteromedial corner, (9) a broaderascending process, and (10) a reduced fibular articulation. (H) Cranial view of the distal tarsals, indicating (11) a proximally flatlateral distal tarsal. All figured material refers to the mounted skeletons (GPIT I and III) of the ‘‘Sauriersaal’’ at Institut fur Geologieund Palaontologie, Tubingen (Weishampel & Westphal, 1986), except: B = SMNS 12949. Scale bars: A = 1 m; B-E, G-H = 5 cm;F = 10 cm.

(1996) for a combined approach. Some (Fraser et al., 2002;Rauhut, 2003; Ezcurra, 2006; Irmis et al., 2007a) adopteda topographic criterion, simply considering the number ofsacral vertebrae, while others (Sereno et al., 1993, Sereno,1999; Langer, 2004; Langer & Benton, 2006, Yates, 2007a)attempted to recognize whether trunk or caudal elementshave been incorporated into the sacrum. Evidence fora two-vertebrae sacrum within basal dinosaurs is limited,and restricted to incomplete specimens (Langer & Benton,2006; Yates, 2007a; Sereno, 2007b). On the contrary, thesacrum of Silesaurus is clearly composed of three sacralvertebrae (Dzik & Sulej, 2007). Accordingly, based on thecurrent evidence, and considering Silesaurus as closely relatedbut outside Dinosauria, the statement that dinosaurs areapomorphic in having a sacrum composed of more thantwo vertebrae is misleading. A more detailed approach thatattempts to recognize trunk or tail additions to the sacrummay provide further information. In a few basal dinosaurs, i.e.Saturnalia tupiniquim, Herrerasaurus ischigualastensis, Staurikosaurus

pricei, Guaibasaurus candelarienesis, and Eoraptor lunensis, the twoprimordial sacral vertebrae are readily recognized basedon their much larger rib articulations. Other vertebraemay be incorporated into the sacrum from either thetrunk (Herrerasaurus, Eoraptor) or the caudal (Staurikosaurus,Saturnalia) series, but none has a conspicuous sacral rib,

compared to the primordial elements. Such a robust thirdelement is known in Silesaurus opolensis, and we agree withDzik & Sulej (2007) that it is borne by a trunk vertebraadded to the sacrum. Among the major dinosaur groups, alltheropods and ornithischians have trunk vertebrae added tothe sacrum, as is also the case in sauropodomorphs, exceptfor Plateosaurus (Yates, 2003c) and, possibly, Thecodontosaurus

(Yates, 2007a). Accordingly, even if a trunk vertebra addedto the sacrum is seen in most basal dinosaurs, the presenceof this character in Silesaurus dismisses its apomorphic statusfor the group. On the other hand, the incorporation ofa caudal vertebra to the dinosaur sacrum seems morerestricted, absent in various basal forms (i.e. Herrerasaurus,

Eoraptor) and most basal sauropodomorphs (Yates, 2007a).Indeed, the presence of caudosacral vertebrae is also notaccepted as a dinosaur apomorphy. It is evident that weare dealing with a highly homoplastic character, possiblyaffected by frame shift phenomena (Galton & Upchurch,2000). It is also of misleading codification if one considers theambiguous condition of vertebrae that bore small transverseprocesses/ribs that attach to the ilium and/or other sacraltransverse processes/ribs; compare Herrerasaurus in Novas(1993) and Sereno (2007b). The increase in the numberof sacral vertebrae is, generally speaking, surely a typicaldinosaur trait. Yet, until more information, possibly derived



from better preserved specimens of key taxa, is available,the number of sacral vertebrae, and also the incorporationof either trunk or caudal elements in the sacrum cannot beunambiguously defined as dinosaur apomorphies. Besides,Langer & Benton (2006) considered a dorsally expandedcranial margin of the first primordial sacral rib as apomorphicfor dinosaurs. Similarly, this condition was also recognized inSilesaurus (ZPAL Ab III/404/3), and can not be considered adinosaur apomorphy in the phylogenetic framework adoptedhere.

Few characters of the pectoral girdle and limb have beenconsidered apomorphic for dinosaurs. This may indicate thatthese parts of the dinosaur skeleton are not very modifiedrelative to the basic archosaur condition. Yet, it may alsoreflect the lack of knowledge regarding these anatomicalelements, especially the forearm and hand, in the outgroupsto Dinosauria. This is particularly the case with the char-acters related to the reduction of the outer digits of thedinosaur manus (Gauthier & Padian, 1985; Novas, 1996;Sereno, 1999). Indeed, dinosaur digit IV is always sube-qual to or shorter than metatarsal III and never possessesmore than three phalanges, none of which is an ungual(Langer & Benton, 2006). In addition, almost no dinosauris known to possess more than two phalanges in manualdigit V. On the contrary, manual digits IV and V of otherarchosauromorphs are elongated elements with three ormore phalanges. More recently, Butler et al. (2007) claimedthat an enlarged grasping manus (with elongated pre-ungualphalanges, prominent dorsal extensor pits and proximalintercondylar processes), previously considered typical of Her-

rerasaurus ischigualastensis and theropods (Sereno et al., 1993;Sereno, 1999), may also be apomorphic for dinosaurs, dueto its occurrence in basal ornithischians (Eocursor parvus andheterodontosaurids). However, the manus is unknown innon-dinosaur dinosauromorphs, and it is ambiguous at whichpoint of basal dinosauromorph evolution these modificationsoccurred. Likewise, although no sternal plates have been rec-ognized in basal dinosauromorphs, this may simply representa preservation bias (Padian, 1997b), and their occurrence aspaired ossifications (Sereno, 1999) can not be regarded as atrustworthy dinosaur apomorphy.

In fact, the single feature of the pectoral skeleton acceptedby most previous studies as apomorphic for Dinosauriaappears to be a long deltopectoral crest (Fig. 5D), whichextends for more than 30-35% of the humeral length.Besides, as noted by several authors (Yates, 2007a; Irmiset al., 2007a; Brusatte et al., 2008a), contrasting with that ofpseudosuchians and Silesaurus opolensis, the deltopectoral crestof dinosaurs is subrectangular, rather than subtriangular orrounded. Yet, although lacking its proximal margin, thedeltopectoral crest of Marasuchus lilloensis (PVL 3871) seemsof the subrectangular type (Bonaparte, 1975), implying amore inclusive distribution for that trait. Likewise, a shorterforearm relative to the humerus can not be accepted a priori

as a dinosaur apomorphy (Irmis et al., 2007a), given thata plesiomorphic longer forearm is retained in Herrerasaurus

ischigualastensis and Eoraptor lunensis (Langer et al., 2007b).

Most novel traits of the early dinosaur skeleton are seenin the pelvic girdle and limb. These were often relatedto the acquisition of an improved bipedal gait (Bakker &Galton, 1974), as typical of most basal members of thegroup. Further, some authors, e.g. Bakker (1971) and Charig(1972, 1984), have suggested that these traits represent keyfeatures that allowed, or even promoted, dinosaur radiationin Late Triassic times, while most other archosaurs were indecline. Regardless of their evolutionary consequences (seeSections IV.2,3), it is true that the dinosaur pelvic girdle andlimb bear various apomorphic traits. Indeed, about half ofthe features presented by Novas (1996) and Sereno (1999)as diagnostic for dinosaurs are related to those elements(exclusive of the sacrum), and similar ratios are seen in otherrecent works: four out of 11 in Langer & Benton (2006);seven out of 11 in Ezcurra (2006); eigth out of 15 in Yates(2007a); and 10 out of 14 in Irmis et al. (2007a). Obviously, thefact that these anatomical parts are relatively well known inbasal dinosauromorphs facilitates the recognition of dinosaurapomorphies.

Regarding the pelvic girdle, a perforated acetabulum(Bakker & Galton, 1974; Novas, 1996; Ezcurra, 2006; Yates,2007a), better described as a straight to concave ventralacetabular margin of the ilium (Langer & Benton, 2006; Irmiset al., 2007a; Brusatte et al., 2008a), stands in most recent revi-sions as a valid synapomorphy of Saurischia plus Ornithischia(Fig. 5E), but that is not the case of a brevis fossa/shelf in theiliac postacetabular ala (Novas, 1996; Sereno, 1999; Fraseret al., 2002; Benton, 2004; Yates, 2007a). Whereas a shelfis also present in Marasuchus (Fraser et al., 2002; Langer &Benton, 2006; but see Novas, 1996), a fossa is not onlyseen in some basal dinosauriforms (e.g. Silesaurus), but is alsolacking in herrerasaurids (Novas, 1992b, 1993, 1996; Langer& Benton, 2006). More recently, Ezcurra (2006) proposeda straight to convex dorsal margin of the ilium (Fig. 5E)as a dinosaur apomorphy. Indeed, basal dinosaurs lack adorsally excavated ilium, which seems to be typical of basaldinosauromorphs (Sereno & Arcucci, 1993, 1994), althoughnot well preserved in some (e.g. Silesaurus; ZPAL AbIII/361).On the contrary, other recently proposed apomorphies ofthe dinosaur ilium are either highly homoplastic (‘‘longpreacetabular process’’; Yates, 2007a) or define a moreinclusive clade (‘‘acetabular antitrochanter present’’; Irmiset al., 2007a), i.e. Dinosauriformes (see Sereno & Arcucci,1994). Irmis et al. (2007a) also suggested that a transverselycompressed distal pubis is a dinosaur apomorphy, reversedin sauropodomorphs, but the definition and distribution ofthis feature is not so straightforward, as extensively discussedby Langer & Benton (2006, p. 338).

Irmis et al. (2007a, char. 73) newly proposed that the pubicprocess of the dinosaur ischium is apomorphic, because‘‘separated from the ilial peduncle’’. In fact, the surfaceconnecting the iliac and pubic articulations of the ischiumis simply excavated in many basal dinosaurs, especiallytheropods (Coelophysis rhodesiensis, QVM QG 1; Liliensternus

liliensterni, MB R 2175) and ornithischians (Scelidosaurus

harrisoni, BMNH1111; Scutellosaurus lawleri, UCMP 130580;



Butler et al., 2007). On the contrary, in forms suchas Marasuchus lilloensis (PVL 3870) and Silesaurus opolensis

(ZPAL AbIII 1228, 404/1) that excavation does notreach the medial-most margin of the ischium, so that amedially displaced sheet of bone remains, filling the spacebetween pubic process and iliac peduncle. This condition isreminiscent of more basal archosaurs, in which the ischiumcontributes significantly to the composition of the medial wallof a non-perforated acetabulum. Herrerasaurus ischigualastensis

(PVL 2566) retains a much reduced medial sheet of bone, sothat the acetabular surface of the ischium can be consideredfully excavated, i.e. bearing the dinosaur apomorphy asdefined by Irmis et al. (2007a). On the contrary, the conditionamong sauropodomorphs is variable (Yates, 2003c); e.g. inSaturnalia tupiniquim (MCP 3846-PV), although an extensiveantitrochanter disrupts the clear observation of the character(but see Liliensternus liliensterni, MB R 2175), the medial sheet ofbone occupies the space between that structure and the pubicarticulation. Accordingly, the status of the character definedby Irmis et al. (2007a) awaits further investigation. Otherpreviously proposed apomorphies of the dinosaur ischiuminclude the presence of a reduced medioventral lamina(Novas, 1996; Langer & Benton, 2006) and a proximaldorsolateral sulcus (Yates, 2007a). Yet, both features areclearly present in Silesaurus (ZPAL AbIII 361, 404/1), sothat their status as apomorphic for dinosaurs depends on thecontentious position of that taxon.

The femur is possibly the most scrutinized bone in the studyof early dinosaurs, with more than ten different charactersfound as apomorphic for the group in the phylogenies revisedhere. An inturned and subrectangular femoral head, that isdistinctly set from the shaft, has been considered amongthe typical traits of dinosaurs by Bakker & Galton (1974)and Gauthier (1986). Yet, this general state was poorlydismembered into distinct and well-defined phylogeneticcharacters, in order to evaluate the apomorphic condition ofeach. Sereno (1999) defined an angular ‘‘greater trochanter’’(i.e. nearly straight angle between the proximal articulationand the long axis of the shaft) as a dinosaur apomorphy,but that trait was also recognized in basal dinosauromorphs(e.g. Pseudolagosuchus major, PULR 53; Ezcurra, 2006). Thisstructures a subrectangular femoral head, if the latter isdistinctly offset from the shaft, as diagnostic of dinosaurs(Ezcurra, 2006, char. 231; Irmis et al., 2007a, char. 81;Brusatte et al., 2008a, char. 132). That condition appearsalong with an inturned femoral head (Fig. 5F), which can bealso considered a dinosaur apomorphy.

Irmis et al. (2007a) claim that the femoral head of dinosaursapomorphicaly bears a ligament sulcus and an asymmetricalfossa articularis antitrochanterica, but these traits have alsobeen recorded in other basal dinosauromorphs (Novas, 1996;Ezcurra, 2006). Likewise, the apomorphic condition of areduced medial tuberosity (Novas, 1996; Sereno, 1999)and a prominent lesser trochanter (Novas, 1996) havebeen dismissed by most recent studies (Langer & Benton,2006; Ezcurra, 2006; Irmis et al., 2007a). Other features ofthe femoral head were considered apomorphic reversals of

Dinosauria (Ezcurra, 2006, char. 232; Irmis et al., 2007a,char. 85; Brusatte et al., 2008a, char. 135), but dependon character optimization. Besides, although reversed intheropods, the presence of an asymmetrical fourth trochanter(Fig. 5F) appears as a valid dinosaur apomorphy in mostrecent reviews (Langer & Benton, 2006; Ezcurra, 2006;Irmis et al., 2007a), and has been recently recorded also inGuaibasaurus candelariensis (Bonaparte et al., 2007; contra Langer& Benton, 2006) and Chindesaurus bryansmalli (GR 226; contra

Yates, 2007a).Previously defined tibial traits such as the presence of a

cnemial crest (Novas, 1996; Sereno, 1999) and a transverselyexpanded distal articulation (Novas, 1996; Benton, 2004)are no longer believed to represent dinosaur apomorphies,given their erratic distribution among basal dinosaurs anddinosauromorphs (Langer & Benton, 2006; Ezcurra, 2006;Irmis et al., 2007a; Brusatte et al., 2008a). Similarly, becausealso seen in Silesaurus, a descending process of the tibia thatcaudally overlaps the ascending process of the astragalusis also not regarded apomorphic for dinosaurs. Accordingto Yates (2007a), a sub-quadratic distal tibia and a thinnerfibula may represent dinosaur apomorphies, because thereverse condition is seen in Silesaurus. Yet, the record ofthe dinosaur condition in Marasuchus lilloensis jeopardizesthat assumption. Accordingly, no unambiguous apomorphyis currently referred to the dinosaur pelvic epipodium. Inaddition, Ezcurra (2006) considered, under DELTRANoptimization, a tibia subequal to the femur as apomorphic fordinosaurs. Although the contrary was described for Silesaurus

opolensis (Dzik, 2003), a longer tibia is not only typical ofbasal dinosauromorphs (Sereno & Arcucci, 1993; 1994;Pesudolagosuchus major, PVL 4629), but was also retained inbasal ornithischians (Santa Luca, 1980; Butler et al., 2007).Indeed, among basal dinosaurs, only saurischians consistentlybear a subequal or longer femur; but see Staurikosaurus pricei

(Colbert, 1970).The tarsal joint has also been the source of several

anatomical traits believed to characterize dinosaurs. Yet,this is not the case of an astragalar ascending process and alateral articulation between the calcaneum and the astragalaranterolateral process (Sereno, 1999), which were recentlyidentified in other basal Dinosauromorpha (Novas, 1996;Langer & Benton, 2006; Brusatte et al., 2008a). Yet, a reducedfibular articulation (Langer & Benton, 2006; Brusatte et al.,2008a), a broader ascending process (Yates, 2007a, char.314), and an acute anteromedial corner (Irmis et al., 2007a)apparently stand as apomorphies of the dinosaur astragalus(Fig. 5G). On the contrary, some putative apomorphiesof the dinosaur calcaneum, such as a concave fibulararticulation (Novas, 1996) and a rudimentary medial process(Sereno, 1999) have an erratic distribution among basaldinosauromorphs, and can not be unambiguously consideredas such (Langer & Benton, 2006; Ezcurra, 2006; Irmis et al.,2007a; Brusatte et al., 2008a). On the other hand, as faras the condition in the outgroups to Dinosauria can beaccessed, a proximally flat lateral distal tarsal (Novas, 1996;Langer & Benton, 2006; Brusatte et al., 2008a) stands as a



unique trait of the group (Fig. 5H). The single apomorphy ofthe dinosaur metatarsus proposed in the discussed studiesof early dinosaur phylogeny is the so-called ‘‘sigmoid’’metatarsal IV (Sereno, 1999), a condition given by thelateral displacement of the distal part of the bone (Novas,1996; Brusatte et al., 2008a). This condition is, however, alsoseen in some basal dinosauromorphs (Novas, 1996; Ezcurra,2006), and disregarded as a dinosaur apomorphy.

(2) Phylogenetic definitions: naming early dinosaurs

With the advent of Phylogenetic Nomenclature (De Queiroz& Gauthier, 1990, 1992, 1994), systematists acquired anunprecedented tool to define taxon names in explicitphylogenetic context, setting their composition accordingto given hypotheses. A drawback of this revolution wasthe inflation of phylogenetic definitions for various names(Benton, 2000), as readily recognized in a brief inspectionof Paul Sereno’s webpage TaxonSearch. Indeed, when dealingwith these names, authors currently have to state whichof the available definitions is adopted to translate theminto the phylogenetic nomenclature system. The priorityissue is expected to be settled with the publication ofthe ‘‘companion volume’’ of the PhyloCode (Cantino & DeQueiroz, 2007). Yet, before this volume is published and,more importantly, accepted by the scientific community asthe ‘‘Systema Naturae’’ of phylogenetic definitions, thesewill no doubt continue to proliferate in an unordered way.In the following paragraphs, the phylogenetic definitionspertinent to the discussion of dinosaur origins are treated inhistorical order and, in an attempt to emulate the ‘‘Principleof Priority’’ (ICZN, 1999), those first proposed, with smallmodifications added if absolutely required, are listed inTable 1 and employed throughout the text.

Because Jacques Gauthier was involved in the studyof archosaurs, including dinosaurs, he presented somephylogenetic definitions for related groups (Gauthier &Padian, 1985; Gauthier, 1986) even before the publication ofthe paper that set the theoretical foundation of PhylogeneticNomenclature (De Queiroz & Gauthier, 1990). Gauthier& Padian (1985) provided a phylogenetic definition forOrnithosuchia, while Gauthier (1986) explicitly definedSaurischia and Theropoda. Problematic aspects of thesedefinitions include the use of supraspecific and/or informalspecifiers (e.g. birds, archosaurs, crocodiles, dinosaurs,sauropodomorphs, Ornithischia) and their choice based onthe phylogenetic orthodoxy of the time. Instead, we believethat, for the sake of precision, newly proposed phylogeneticdefinitions should use minimal groups as specifiers, andfor historical coherence rely, as much as possible, on taxamentioned in the original definition of the names. In any case,because first published, those definitions are adopted herefor the names in question (Table 1). Alternative phylogeneticdefinitions for Saurischia (Padian & May, 1993; Padian,1997d; Sereno, 1998; Holtz & Osmolska, 2004; Langer,2004) just replace specifiers, either because these are morespecific (Padian, 1997d; Sereno, 1998) or are quoted in theoriginal proposition of the name (Langer, 2004). Yet, based

on current phylogenetic hypotheses, these circumscribe thesame set of taxa as Saurischia sensu Gauthier (1986). Similarly,alternative specifiers in later definitions of Theropoda aremore specific (Currie, 1997) and either more highly nested(Sereno, 1998) or first named (Padian, Hutchinson & Holtz,1999; Holtz & Osmolska, 2004). Again, their use does notchange the inclusivity of the group as defined by Gauthier(1986).

Further phylogenetic definitions pertinent to the discussedgroups were proposed by Sereno (1991a), Novas (1992b),and Padian & May (1993). Sereno (1991a) gave node-based definitions for Ornithodira Gauthier, 1986, andDinosauromorpha Benton, 1985. These had to be slightlymodified (Table 1) to fit the logical basis of PhylogeneticNomenclature and the updated taxonomy of Sereno &Arcucci (1994), but substitute definitions (Benton, 2004)are redundant. Especially problematic are the stem-based definitions of Dinosauromorpha (Sereno, 1991a,2005; Benton, 2004) that use pterosaurs as the externalspecifier, given the uncertain phylogenetic position of thesereptiles. In their current understanding, Ornithodira andDinosauromorpha differ only by the inclusion of Scleromochlus

taylori and possibly pterosaurs in the former. The leastinclusive Dinosauriformes was node-based defined whenfirst named by Novas (1992b). This was modified (Table 1)to fit the taxonomy of Sereno & Arcucci (1994), but equallyrequires no substitute definitions (Benton, 2004).

Apart from the equivocal list of taxa presented by Gauthier(1986, p. 44; see Padian, 1997a), Novas (1992b) provided thefirst phylogenetic definition of Dinosauria as ‘‘the commonancestor of Herrerasauridae and Saurischia + Ornithischia,and all of its descendants’’. This is in agreement withthe taxonomic orthodoxy of the time (Gauthier, 1986;Brinkman & Sues, 1987; Benton, 1990; but see Gauthieret al., 1989), according to which: (1) saurischians plusornithischians form a clade, contrary to the traditional viewthat these arose independently from ‘‘thecodont’’ precursors;(2) herrerasaurids, conventionally regarded as saurischiandinosaurs (Reig, 1963; Colbert, 1970), are basal to thatclade. Indeed, in order to keep herrerasaurids as dinosaurs,Novas (1992b) used the former as an internal specifier ofthe latter. By contrast, Padian & May (1993) explicitlyrestricted the use of Dinosauria to the clade composed ofSaurischia and Ornithischia, exclusive of ‘‘Herrerasaurus andits allies’’. Despite the ‘‘priority’’ of Novas (1992b), the latterconcept gained almost unconditional acceptance since (e.g.Sereno, 1998, 2005; Fraser et al., 2002) and is employedhere (Table 1). In any case, these alternate definitionsonly circumscribe different groups if herrerasaurids areplaced outside the Saurischia + Ornithischia dichotomy,a hypothesis not supported by most recent studies (seebelow). Other authors (Holtz in Padian, 1997a; Olshevsky,2000; Clarke, 2004) attempted phylogenetically to defineDinosauria using taxa included in the original propositionof the name. In this case, the best option may be using allnames mentioned by Owen (1842) in a node-based fashion,and to define Dinosauria as ‘‘the most recent common



Table 1. Phylogenetic definition of names relevant in the context of early dinosaur evolution.

Name Phylogenetic definition

ORNITHODIRAGauthier, 1986

‘‘Pterosauria, Scleromochlus, Dinosauromorpha (including birds), and all descendantsof their most recent common ancestor’’ modified from Sereno (1991a),node-based

DINOSAUROMORPHABenton, 1985

‘‘Lagerpeton chanarensis, Marasuchus lilloensis, Pseudolagosuchus major, Dinosauria (inc.Aves), and all descendants of their most recent common ancestor’’ modified fromSereno (1991a); node-based

DINOSAURIFORMESNovas, 1992b

‘‘The most recent common ancestor of Marasuchus lilloensis, Dinosauria, and all taxastemming from it’’ modified from Novas (1992b); node-based

SILESAURIDAEnew name

‘‘All archosaurs closer to Silesaurus opolensis, than to Heterodontosaurus tucki andMarasuchus lilloensis’’; stem-based

DINOSAURIAOwen, 1842

‘‘All descendants of the most recent common ancestor of birds and Triceratops’’Padian & May (1993); node-based

ORNITHISCHIASeeley, 1888

‘‘Dinosaurs closer to Triceratops than to birds’’ Padian & May (1993); stem-based

GENASAURIASereno, 1986

‘‘Thyreophora and Cerapoda and all descendants of their common ancestor’’Currie & Padian (1997a); node-based

NEORNITHISCHIACooper, 1985

‘‘All genasaurs closer to Triceratops than to Ankylosaurus’’ Sereno (1998); stem-based

THYREOPHORANopcsa, 1915

‘‘All genasaurs closer to Ankylosaurus than to Triceratops’’ Sereno (1998); stem-based

SAURISCHIASeeley, 1888

‘‘Birds and all dinosaurs that are closer to birds than they are to Ornithischia’’Gauthier (1986); stem-based

HERRERASAURIAGalton, 1985b

‘‘All dinosaurs that share a more recent common ancestor with Herrerasaurus thanwith Liliensternus and Plateosaurus’’ Langer (2004); stem-based

HERRERASAURIDAEBenedetto, 1973

‘‘Herrerasaurus, Staurikosaurus, their most recent common ancestor, plus all itsdescendants’’ modified from Novas (1992b); node-based

EUSAURISCHIAPadian et al. 1999

‘‘The least inclusive group of Saurischia, containing Cetiosaurus and Neornithes’’Langer (2004); node-based

SAUROPODOMORPHAHuene, 1932

‘‘The clade including the most recent common ancestor of Prosauropoda andSauropoda and all of its descendants’’ Salgado et al. (1997); node-based

MASSOPODAYates, 2007a

‘‘The most inclusive clade containing Saltasaurus loricatus but not Plateosaurus

engelhardti’’ Yates (2007a); stem-basedSAUROPODIFORMES

Sereno, 2005‘‘The least inclusive clade containing Mussaurus patagonicus Bonaparte & Vince,

1979, and Saltasaurus loricatus Bonaparte & Powell, 1980’’ Sereno (2005);node-based

SAUROPODAMarsh, 1878

‘‘The most recent common ancestor of Vulcanodon karibaensis and Eusauropoda andall of its descendants’’ Salgado et al. (1997); node-based

THEROPODAMarsh, 1881

‘‘Birds and all saurischians that are closer to birds than they are tosauropodomorphs’’ Gauthier (1986); stem-based

NEOTHEROPODABakker, 1986

‘‘Coelophysis, Neornithes, their most recent common ancestor and all descendants’’Sereno (1998); node-based

COELOPHYSOIDEANopcsa, 1928

‘‘All ceratosaurs closer to Coelophysis than to Carnotaurus’’ Sereno (1998);stem-based

AEROSTRAPaul, 2002

‘‘Ceratosaurus nasicornis, Allosaurus fragilis and all the descendants of their most recentcommon ancestor’’ modified from Ezcurra & Cuny (2007); node-based

ancestor of Megalosaurus, Iguanodon, and Hylaeosaurus, and all itsdescendants’’. Again, according to the current phylogenetichypotheses, this definition circumscribes the same set of taxaas that of Padian & May (1993).

Novas (1992b) also proposed a node-based definition forHerrerasauridae, ‘‘emended’’ by Novas (1997a). Yet, bothdefinitions are incomplete and a modified version of them isemployed here (Table 1). There is no good reason to replacethat definition with a stem-based Herrerasauridae (Sereno,1998; Benton, 2004), especially because this is equivalent toHerrerasauria (see below). Further, Padian & May (1993)

provided a stem-based definition for Ornithischia, in afashion that matches its mutual exclusivity in relation toSaurischia sensu Gauthier (1986). Subsequent definitions usemore specific (Sereno, 1998) and also more ‘‘traditional’’(Weishampel, 2004; Norman, Witmer & Weishampel, 2004a)specifiers, but are equally inclusive based on currentphylogenies. Although the use of taxa mentioned in theproposal of Saurischia (e.g. Allosaurus, Camarasaurus) andOrnithischia (e.g. Stegosaurus, Iguanodon) may have been moredesirable, all the previous definitions successfully translateSeeley’s (1888) dichotomous understanding of Dinosauria



into the Phylogenetic Nomenclature system. Likewise, itcould also be argued that the use of apomorphy-baseddefinitions for Saurischia and Ornithischia better representsthat original proposition, given that the groups were definedon a character basis, i.e. opisthopubic and propubic pelves.Yet, this is problematic because only the ornithischian pelvicconstruction is apomorphic, whereas saurischians retain thegeneral morphology seen in more basal archosaurs.

Salgado, Coria & Calvo (1997) first proposed aphylogenetic definition for Sauropodomorpha (Table 1).Their node-based definition preceded that (stem-based)given by Upchurch (1997b) by a couple of months, butboth suffer from using Sauropoda and Prosauropoda asinternal specifiers. Subsequent proposals attempt to replacethose taxa by more specific, and deeply nested specifiers ineither a node- (Sereno, 1998) or stem- (Galton & Upchurch,2004; Sereno, 2007a) based fashion. Although lower rankspecifiers are desirable, the same level of precision can beachieved using higher taxa that are, in turn, defined withdirect reference (or by typification) to those minimal groups.Moreover, the adequacy of an either stem- or node-basedSauropodomorpha (Upchurch, Barrett & Galton, 2007) isminor in face of the primacy of the definition provided bySalgado et al. (1997).

More recently, Langer (2004) defined a stem-basedHerrerasauria Galton, 1985b, and a node-based EusaurischiaPadian, Hutchinson & Holtz, 1999. The former group ispotentially equivalent to Herrerasauridae sensu Sereno (1998),but the node-based original definition of Herrerasauridae isemployed here. In that context, Herrerasauria (Table 1) canallocate dinosaurs closely related to, but outside the cladecomposed of Herrerasaurus plus Staurikosaurus. Eusaurischia,on the other hand, was first proposed to designate the cladecomposed of Sauropomorpha plus Theropoda (Padian et al.,1999). This is as inclusive as the stem-based Saurischia undercertain phylogenetic schemes (Novas, 1996; Sereno, 1999),but excludes basal forms such as Eoraptor and herrerasaursin alternative frameworks (Langer, 2004; Ezcurra, 2006)and remains a potentially useful name (Table 1). Finally,Silesauridae is here defined as a stem-based taxon thatincludes all archosaurs closer to Silesaurus opolensis than toMarasuchus lilloensis and Heterodontosaurus tucki. The latterform was chosen to represent Dinosauria because of itscompleteness (Santa Luca, 1980) and basal position withinOrnithischia (Butler et al., 2007), a group to which Silesaurus

has been tentatively related (Ferigolo & Langer, 2007).

III. DINOSAUR ‘‘TRAIL BLAZERS’’ IN SPACE,TIME, AND EVOLUTIONARY CONTEXT

(1) The oldest dinosaurs and the rocks that containthem

For most of the last century, except in a few importantcases (Huene, 1926; Colbert, 1989; Sereno & Novas, 1992;Sereno et al., 1993), the knowledge of Triassic dinosaurs was

based on incomplete and/or fragmentary skeletal remains.In the last decade, however, various studies (e.g. Rauhut& Hungerbuhler, 2000; Langer, 2004; Parker et al., 2005;Ezcurra, 2006; Nesbitt et al., 2007) revised those early records,questioning the dinosaur affinity of several of them. On theother hand, the discovery of a variety of more completebasal dinosaurs (e.g. Langer et al., 1999; Bonaparte et al.,1999, 2007; Yates & Kitching, 2003; Butler et al., 2007; Pol& Powell, 2007a, b; Martinez & Alcober, 2009; Ezcurra,2008), allowed a more reliable picture to emerge. As detailedbelow, this accounts for the possible, but poorly supportedMiddle Triassic origin of the group, its first radiation duringthe Carnian, and the full establishment of the main dinosaurgroups from the Norian onwards.

Usually, the oldest dinosaurs (Galton, 2000; Langer,2004) are considered as coming from the Ischigualastianbeds (Langer, 2005a) of northwestern Argentina and southBrazil (Fig. 6). These respectively include the IschigualastoSequence, Ischigualasto-Ischichuca depocenter, BermejoBasin (Stipanicic & Marsicano, 2002; Currie et al., 2009),and the Santa Maria Supersequence, Parana Basin (Zerfasset al., 2003), the continental sedimentation of which filledextensional rift basins related to the Gondwanides orogenesis(Zerfass et al., 2004). Early works dated the Ischigualastoand Santa Maria formations as Middle Triassic (Romer,1960, 1962; Reig, 1961, 1963), but a Late Triassic age,first proposed by Bonaparte (1966), has been supported bymost recent biostratigraphic studies (Ochev & Shishkin, 1989;Lucas, 1998; Langer, 2005a, b). This was corroborated by theradiometric dating of the ‘Herr Toba’ bentonite (Fig. 6C),at the base of the Ischigualasto Formation (Rogers et al.,1993), that provided a 40Ar/39Ar age of 227 ± 0.3 Mya. Yet,following the discrepancy between U-Pb and 40Ar/39Ar dates(Schoene et al., 2006) and other comparative parameters,Furin et al. (2006) recalculated a date of 230.3-231.4 ± 0.3Mya. This corresponds to the late Ladinian in most timescales(Ross, Baud & Manning, 1994; Remane et al., 2000; Ogg,2004; Ogg, Ogg & Gradstein, 2008), but recent works(Muttoni et al., 2001, 2004; Gallet et al., 2003; Kent, Muttoni& Brack, 2006; Kozur & Weems, 2007) assigned older agesfor the Carnian boundaries. In that context, and consideringthe sedimentation rate of comparable rift basins (Rogerset al., 1993; Currie et al., 2009), the dinosaur-rich sites of thelower third of the Ischigualasto Formation can be placed inthe latest Carnian. Yet, the middle third of that stratigraphicunit, that also yielded dinosaur remains, may rest withinthe middle Norian. This was recently corroborated by thedating of another bentonite, from above the middle sectorof the Ischigualasto Formation (Currie et al., 2009), whichprovided a 40Ar/39Ar age of 217.0 ± 1.7 Ma (Shipman,2004), recalculated as 219.4-220.4 ± 1.7 Mya (M. Ezcurra,personal observations).

Ischigualastian dinosaurs (Fig. 6C) include Herrerasaurus

ischigualastensis, along with its possible synonyms Ischisaurus

cattoi and Frenguellisaurus ischigualastensis (Novas, 1993), Eoraptor

lunensis (Sereno et al., 1993), and Panphagia protos (Martinez& Alcober, 2009), from the lower third of the Ischigualasto



Fig. 6. Tectonic and sedimentary settings of southwestern Pangea during the Middle and Late Triassic, with emphasis on the SouthAmerican dinosaur-bearing sequences (Zerfass et al., 2004; Veevers, 2005). (A) Idealized east-west cross section from Santa Mariaintraplate rift to the Cuyo back-arc rift and Gondwanides orogen. (B) Palaeogeographic reconstruction; note that the extensionalbasins are perpendicular to the transtensional stresses. Abbreviations as follows: SLV, Sierra de la Ventana; CFB, Cape Fold Belt.Gondwanides orogen in grey. (C) Stratigraphic charts of the Bermejo and Parana Basins, depicting the dinosauromorph/putativedinosaur record. Fm., Formation; HAZ, Hyperodapedon Acme Zone according to Langer et al. (2007c); Mys, million years beforerecent. Asterisks indicate possibly coeval faunas in which the dicynodont Jachaleria occurs.

Formation, and Pisanosaurus mertii (Bonaparte, 1976) from themiddle third of that stratigraphic unit (Rogers et al., 1993),as well as Staurikosaurus pricei (Colbert, 1970) and Saturnalia

tupiniquim (Langer et al., 1999) from the Hyperodapedon

Assemblage-Zone of the Santa Maria Formation (Langeret al., 2007b). More recently, the discoveries of two newherrerasaurids (Martinez & Alcober, 2007; Ezcurra & Novas,2008), a Saturnalia-like animal (Ezcurra & Novas, 2008;Ezcurra, 2008), and a probable basal theropod (Martinez,Sereno & Alcober, 2008) have been announced from theIschigualasto Formation. Outside South America, dinosaursof similar age are much less conspicuous (Fig. 7). Thesemainly include fragmentary remains from Gondwanan areassuch as the possible record of Saturnalia in the Pebbly ArkoseFormation (Cabora Bassa Basin), lower Zambezi Valley,Zimbabwe (Raath, 1996; Langer et al., 1999), and partof the specimens attributed to Alwalkeria maleriensis, fromthe Lower Maleri Formation (Pranhita-Godavari Basin), incentral Peninsular India (Chatterjee, 1987; Remes & Rauhut,2005). The record of dinosaurs in other coeval deposits

such as the Timesgadiouine Formation (Argana Basin),in Morocco (Jalil, 1996, Gauffre, 1993), and the Isalo IIbeds (Morondava Basin), in Madagascar (Flynn et al., 1999),has been dismissed (Jalil & Knol, 2002; Flynn et al., 2008).According to Langer (2005b) the Ischigualastian can betraced into northern Pangea to encompass the LossiemouthSandstone Formation, in northern Scotland. Yet, the onlyputative dinosaur from those strata, Saltopus elginensis, hasdoubtful affinities to the group (Rauhut & Hungerbuhler,2000; Langer, 2004).

All dinosaur osteological records from pre-Ischigualstianstrata have been questioned, including Spondylosoma abscon-

ditum (Galton, 2000; Langer et al., 2007c), from the SantaMaria 1 sequence in south Brazil (Fig. 6C). Further occur-rences of the group in strata of equivalent age, mainlybased on fragmentary European specimens (Huene, 1932),have also been dismissed (Benton, 1986b; Norman, 1990;Galton & Walker, 1996; Rauhut & Hungerbuhler, 2000).On the other hand, suggestions that dinosaurs were alreadypresent in Middle Triassic times are backed up by two lines



Fig. 7. Distribution of the main tetrapod-bearing deposits of Late Triassic age and their dinosaur record. (A) Chinle Formation andDockum Group, western USA; (B) Newark Supergroup, North American Atlantic coast; (C) Argana Basin, Morocco; (D) JamesonLand, Greenland; (E) Fissure-filling and Rhaetian deposits, northwestern Europe; (F) Germanic Basin, Central Europe; (G) KhoratPlateau, Thailand; (H) Bermejo Basin, Argentina; (I) El Tranquilo Group, Argentina; (J) Parana Basin, Brazil; (K) Karoo basins,south-central Africa; (L) Morondava Basin, Madagascar; (M) Pranhita-Godavari Basin, India. Late Triassic map redrawn fromBlakey (2006). Generalized black silhouettes (not at the same scale) adapted from various sources. Fm., Formation; Mb., Member;Mbs, members.

of evidence: trackways and the stratigraphic calibration ofphylogenetic hypotheses. Indeed, if silesaurids are acceptedas an inclusive sister taxon to Dinosauria (Nesbitt et al., 2007,p. 214; Brusatte et al., 2008a; contra Ezcurra, 2006), encom-passing Middle Triassic forms such as Pseudolagosuchus andLewisuchus, then the dinosaur stem (although not necessarilydinosaurs) minimally arose at the same time, i.e. the LadinianStage. This is supported by evidence extrapolated from thepalaeoichnological record. Tracks suggest the presence ofdinosauromorphs in the Middle Triassic of France (Lockley &Meyer, 2000), Italy (Avanzini, 2002), and Germany (Haubold& Klein, 2002). Some German tracks may correspond todinosaurs, as is also the case for Middle Triassic footprintsfrom various stratigraphic units in Argentina (Melchor & DeValais, 2006; Marsicano, Domnanovich & Mancuso, 2007),including the Los Rastros Formation (Fig. 6C). Althoughthese may also represent basal dinosauriforms, the already

diversified and somewhat advanced fauna of saurischiansfound in the superposed Ischigualasto Formation, providessome basis to infer a Middle Triassic origin of dinosaurs.

In the scheme proposed by Langer (2005b), some tetrapodassemblages of the Newark Supergroup (Olsen, Schlische& Gore, 1989), in the North American Atlantic coast(Fig. 7), albeit slightly younger than those of the Ischigualastoand Santa Maria formations, may correspond to the LateIschigualastian. These include the faunas of the Wolfville(Fundy Basin, Nova Scotia) and Pekin (Deep River Basin,North Carolina) formations, the ornithischian records ofwhich (Galton, 1983b; Hunt & Lucas, 1994) were consideredunsubstantiated by Irmis et al. (2007b). In any case, duringpost-Ischigualastian times, dinosaurs became more abundantand widespread. Some of these taxa have been known forover a century (Meyer, 1837; Cope, 1889), but the diver-sity of Norian dinosaurs (Fig. 8) was greatly enhanced by



Fig. 8. Skeletal reconstructions (from various sources), at approximately the same scale, of selected Carnian and Norian dinosaurs,partially depicting the Late Triassic diversity of the group. Scale bar (lower left ) = 1 m.

last-decade discoveries, especially from South America andSouth Africa, as outlined below.

Post-Ischigualastian dinosaur faunas in South Americainclude those of the Los Colorados, Laguna Colorada, andCaturrita formations (Langer, 2005a). The latter strati-graphic unit, in south Brazil (Fig. 6C), has yielded thesaurischian Guaibasaurus candelariensis (Bonaparte et al., 1999,2007), as well as the ‘‘prosauropod’’ Unaysaurus tolentinoi

(Leal et al., 2004). ‘‘Prosauropods’’ are well known in the LaEsquina fauna of the Los Colorados Formation (Bonaparte,1972). That stratigraphic unit covers the Ischigualasto For-mation in northwestern Argentina (Fig. 6C), and includesRiojasaurus incertus (Bonaparte & Plumares, 1995), Coloradis-

aurus brevis (Bonaparte, 1978), and Lessemsaurus sauropoides (Pol& Powell, 2007a), along with theropods (Bonaparte, 1972)such as Zupaysaurus rougieri (Arcucci & Coria, 2003; Ezcurra& Novas, 2007a). In Patagonia, the Laguna Colorada For-mation (El Tranquilo Group) has yielded the ‘‘prosauropod’’Mussaurus patagonicus (Bonaparte & Vince, 1979; Pol & Powel,2007b) as well as a heterodontosaurid ornithischian (Baez& Marsicano, 2001). Other dinosaur-bearing gondwanandeposits of similar age (Fig. 7) include the Lower Elliot For-mation (Stormberg Group, Karoo Basin), in South Africa

(Knoll, 2005), and the Upper Maleri Formation, in penin-sular India. The latter, along with the overlying LowerDharmaram Formation, has yielded a diversified, but stillundescribed fauna of basal saurischians (Kutty & Sengupta,1989; Novas et al., 2006), which may include a Guaibasaurus-like form (Kutty et al., 2007). Basal sauropodomorphs arealso well known in the Lower Elliot Formation, whereMelanorosaurus readi, Antetonitrus ingenipes, Blikanasaurus cromp-

toni, Eucnemesaurus fortis, Plateosauravus cullingworthi, and a yetunnamed form (Yates, 2003a, 2007a, b, 2008; Yates &Kitching, 2003) were recorded along with the ornithischianEocursor parvus (Butler et al., 2007) and possible theropod teeth(Ray & Chinsamy, 2002).

In North Pangea, various Norian faunas of Europe andNorth America yielded dinosaur records (Fig. 7). Theseinclude the rich prosauropod fauna of the German Keuper,where Efraasia minor occurs in the Middle Stubensand-stein (Lowenstein Formation) of Baden-Wurttemberg, alongwith Procompsognathus triassicus and other possible theropods(Hungerbuhler, 1998; Rauhut & Hungerbuhler, 2000; Yates,2003a). Specimens/species attributed to Plateosaurus aremuch more widespread both geographically and stratigraph-ically (Yates, 2003c; Moser, 2003; Weishampel et al., 2004),



also occurring in the overlying Knollenmergel (Trossin-gen Formation, and related stratigraphic units) of Baden-Wurttemberg, Bavaria, Lower Saxony, and Saxony-Anhalt,as well as in putative coeval faunas from France, Switzerland,and Greenland (Jenkins et al., 1994; Galton & Upchurch,2004). In addition, the Thuringian Knollenmergel hasyielded the ‘‘prosauropod’’ Ruehleia bedheimensis and the thero-pod Liliensternus liliensterni (Rauhut & Hungerbuhler, 2000;Galton, 2001). The ‘‘lower’’ fissure-filling deposits of theBritish Isles (southwest England and south Wales) are alsofrequently regarded as Norian in age (Fraser, 1994; Ben-ton & Spencer, 1995), although they might well spreadinto the late Carnian and/or Rhaetian (Benton et al., 2000).Among these, the Pant-y-ffynnon site, in south Wales, isbetter known for its dinosaur fauna, which includes thebasal sauropodomorph Pantydraco caducus (Kermack, 1984;Yates, 2003b; Galton, Yates & Kermack, 2007) and a smalltheropod possibly related to Coelophysis/Syntarsus (Rauhut &Hungerbuhler, 2000). P. caducus was previously assigned tothe genus Thecodontosaurus, the type species of which (T.

antiquus) is also known from various other putatively coevalfissure-filling deposits (Benton & Spencer, 1995; Bentonet al., 2000). In addition, the Cromhall Quary, in Avon, hasyielded the specimens assigned to Agnosphitys cromhallensis, thedinosaur affinity of which is controversial (Fraser et al., 2002;Langer, 2004; Yates, 2007a). Perhaps, the youngest dinosaur-bearing deposits of the European Triassic are the Rhaetianbeds of Normandy (northern France), Somerset-Avon (south-west England), Mid-Glamorgan (south Wales), and Belgium.These include the indeterminate theropod ‘‘Zanclodon’’ cam-

brensis (Rauhut & Hungerbuhler, 2000; Galton, 2005a), thecoelophysoid Lophostropheus airelensis (Ezcurra & Cuny, 2007),sauropodomorphs like Camelotia borealis (Storrs, 1994; see alsoGodefroit & Knoll, 2003), and the very unlikely record ofa stegosaur (Galton, 2005a; Irmis et al., 2007b). In addi-tion, a possible theropod has been recovered recently fromthe Rhaetian beds of Lipie Slaskie, Poland (Dzik, Sulej& Niedzwiedzki, 2008). Isolated ‘‘dinosaur’’ teeth, mainlyassigned to Ornithischia, have also been reported exten-sively from Norian-Rhaetian European strata (Weishampelet al., 2004), none of which was recently confirmed (Butler,Porro & Heckert, 2006; Irmis et al., 2007b). The ‘‘Eurasian’’record of Norian-Raethian dinosaurs (Fig. 7) is completed bythe basal sauropodomorphs of the Nam Phong Formation,Thailand, that include Isanosaurus attavipachi (Buffetaut et al.,1995, 2000).

The record of Triassic dinosaurs in western USA wasrecently reviewed by Nesbitt et al. (2007; see also Parkeret al., 2005; Ezcurra, 2006; Nesbitt & Chatterjee, 2008).No compelling evidence of either sauropodomorphs orornithischians was found, and only coelophysoids werepositively identified, along with putative basal saurischians(herrerasaurs) and basal theropods (Fig. 7). Given thatthe Santa Rosa Formation ‘theropod’ (Heckert, Lucas &Sullivan, 2000) was considered an indeterminate archosaur(Nesbitt et al., 2007), the oldest dinosaur from westernUSA, and possibly the oldest known neotheropod so far

is ‘‘Camposaurus arizonensis’’, an indeterminate coelophysoidfrom the Placerias Quarry (Bluewater Creek Member, base ofthe Chinle Formation), northern Arizona (Hunt et al., 1998).Younger records of coelophysoids include Coelophysis bauri

(Colbert, 1989; Colbert et al., 1992; ICZN, 1996; Spielmannet al., 2007), the material described by Cope (1889) andPadian (1986), as well as other specimens (Ezcurra, 2006;Irmis et al., 2007a; Spielmann et al., 2007), along with some ofthose attributed to Gojirasaurus quayi (Carpenter, 1997; Nesbittet al., 2007). All these come from Norian deposits referredto the Chinle Formation (Petrified Forest Member and‘‘siltstone member’’), in central New Mexico and Arizona,and the Bull Canyon Formation (Dockum Group), in eastNew Mexico and west Texas (Nesbitt et al., 2007). Amongnon-theropod dinosaurs, whereas Caseosaurus crosbyensis (Huntet al., 1998) was regarded as an indeterminate dinosauriform(Nesbitt et al., 2007), putative herrerasaurs occur in thePetrified Forest Member (Chindesaurus bryansmalli) in Arizona,as well as in the Bull Canyon Formation, which also yieldeda putative basal theropod (Nesbitt et al., 2007; Nesbitt &Chatterjee, 2008). In terms of age, except for those of thePlacerias Quarry, all reliable dinosaur occurrences in theTriassic of western North America are considered youngerthan the Blue Mesa Member of the Chinle Formation, inArizona (Nesbitt et al., 2007), which has been radiometricallydated as 219.2 ± 0.7 Myr (Irmis & Mundil, 2008).

In conclusion, although a Middle Triassic (Ladinian) originof dinosaurs might be hypothesized, the oldest definitiverecords of the group date from about 230 million years ago.This corresponds to the Carnian stage of the Late Triassic.Radiometric dating of different levels of the IschigualastoFormation, Argentina (Rogers et al., 1993; Shipman, 2004)suggests that after about 20 million years, i.e. within the latestTriassic, a more diverse (Fig. 8), and specially more abundantand widespread dinosaur fauna was already present (Benton,1983a; Ezcurra & Novas, 2008), as represented by the LosColorados Formation and correlated assemblages from otherparts of the world (Fig. 7).

(2) The evolutionary tree of early dinosaurs

‘‘Early dinosaurs’’ are broadly understood here as all putativerepresentatives of the group collected from Ischigualastianstrata, as well as younger dinosaurs, the position of whichwithin Ornithischia, Theropoda, or Sauropodomorpha, isyet to be firmly established (Table 2). These include reason-ably well-known forms such as Herrerasaurus ischigualastensis,Pisanosaurus mertii, Staurikosaurus pricei, Eoraptor lunensis, Saturna-

lia tupiniquim, and Panphagia protos, as well as more fragmentarytaxa (Huene, 1910, 1942; Chatterjee, 1987; Long & Murry,1995; Bonaparte et al., 1999; Fraser et al., 2002; Langer,2004; Nesbitt et al., 2007). Pisanosaurus has always been con-sidered an ornithischian dinosaur (Thulborn, 1971; Galton,1972; Bonaparte, 1976), while Herrerasaurus and Staurikosaurus

were assigned into the base of Saurischia by pre-cladisticworks (Reig, 1963; Benedetto, 1973; Galton, 1977), althoughmore specific affinities to either sauropodomorphs (Reig,1970; Colbert, 1970; Van Heerden, 1978) or theropods



Table 2. Taxonomic assignment of ‘‘early dinosaurs’’, as recently given by different authors.

Taxon Proposed affinity

Agnosphitys cromhallensis Non-dinosaur; Fraser et al. (2002)Dinosauria (partim); nomen dubium; Langer (2004)Basal Theropoda; Yates (2007a)Basal Sauropodomorpha (Guaibasauridae); Ezcurra (2008)

Aliwalia rex Eucnemesaurus fortis; Yates (2007a)Alwalkeria maleriensis Basal Saurischia (partim); Remes & Rauhut (2005)Caseosaurus crosbyensis Dinosauriformes Nesbitt et al. (2007)Chindesaurus bryansmalli Herrerasauridae Irmis et al. (2007a)

Basal Theropoda; Yates (2007a)Basal Saurischia (partim); Nesbitt et al. (2007)

Eoraptor lunensis Basal Theropoda; Sereno (1999); Ezcurra (2006)Basal Saurischia; Langer (2004); Yates (2005)

Guaibasaurus candelariensis Basal Saurischia (Guaibasauridae); Bonaparte et al. (2007)Basal Theropoda; Yates (2007a), Langer et al. (2007a)Basal Sauropodomorpha; Ezcurra (2008)

Herrerasauridae Basal Theropoda; Novas (1996); Sereno (1999)Non-dinosaur; Fraser et al. (2002)Basal Saurischia, Langer (2004); Yates (2005); Ezcurra (2006)

Herrerasaurus ischigualastensis Herrerasauridae Novas (1992b)Panphagia protos Sauropodomorpha Martinez & Alcober (2009)Pisanosaurus mertii Ornithischia Sereno (1999); Butler et al. (2007)Saltopus elginensis Dinosauriformes Rauhut & Hungerb uhler (2000); Langer (2004)Saturnalia tupiniquim Stem-Sauropodomorpha; Langer & Benton (2006)

Basal Saurischia (Guaibasauridae); Bonaparte et al. (2007)Sacisaurus agudoensis cf. Ornithischia; Ferigolo & Langer (2007)

Non-dinosaur; Brusatte et al. (2008a)Silesaurus opolensis cf. Ornithischia; Ferigolo & Langer (2007)

Non-dinosaur; Langer & Benton (2006)Spondylosoma absconditum Non-dinosaur; Galton (2000)

cf. Herrerasauridae; Langer (2004)Staurikosaurus pricei Herrerasauridae Novas (1992b)Teyuwasu barberenai Dinosauria (partim); nomen dubium; Langer (2004)

(Galton, 1973; Bakker & Galton, 1974) were also claimed.On the contrary, early cladistic studies (Fig. 9) depictedStaurikosaurus and Herrerasaurus basal to the Ornithischia+Saurischia dichotomy (Gauthier, 1986; Brinkman & Sues,1987; Benton, 1990; Novas, 1992b), thus outside Dinosauriaon its emerging monophyletic understanding (Gauthieret al., 1989), whereas contemporaneous studies never ques-tioned the ornithischian affinity of Pisanosaurus (Novas, 1989;Sereno, 1991b). These investigations set the basis to futureresearch on basal dinosaur phylogeny, accepting the groupas a monophyletic entity solely composed of Ornithischiaand Saurischia, the latter including equally monophyleticSauropodomorpha and Theropoda. Besides, Novas (1992b)placed Herrerasaurus and Staurikosaurus into a monophyleticHerrerasauridae, a hypothesis almost never contested since.

During the early nineties, new discoveries from theIschigualasto Formation, including almost complete skele-tons of Herrerasaurus ischigualastensis (Sereno & Novas, 1992,1993; Novas, 1993, Sereno, 1993) and the first record ofEoraptor lunensis (Sereno et al., 1993), were announced alongwith a new hypothesis of basal dinosaur relationships. Thiswas advocated based on independent numerical analysesperformed by Sereno et al. (1993; see also Sereno, 1999)

and Novas (1996) that found nearly identical results (Fig. 9).The Herrerasauridae was depicted as the sister-taxon ofNeotheropoda, while Eoraptor was considered the basal-mosttheropod. Apomorphic traits supporting the theropod affin-ity of Eoraptor and Herrerasauridae were given as includingcaudally curved tooth crowns not expanded at the base, abroad axial intercentrum, elongated humerus and manus,deep extensor pits on the distal end of metacarpals I–III,and narrow metacarpal IV, as well as by typical preda-tory adaptations shared by herrerasaurids and theropods(Fig. 10), e.g. intramandibular joint, craniomandibular jointat about the same level as the tooth rows, and manualdigits II and III with elongated penultimate phalangesand strongly curved unguals with enlarged flexor tubercles(Langer & Benton, 2006; Sereno, 2007b; but see Butler et al.,2007). More recently, Chindesaurus bryansmalli was describedas a herrerasaurid (Long & Murry, 1995; Novas, 1997a;Sereno, 1999), a phylogenetic hypothesis accepted by mostauthors up to the late nineties. However, the suggestions thatChindesaurus forms a clade with either Herrerasaurus (Novas,1997a) or Staurikosaurus (Sereno, 1999) were not supported byrecent studies (Langer, 2004; Nesbitt et al. 2007; Bittencourt



Fig. 9. Main alternative phylogenetic hypotheses depicting the interrelationships of ‘‘early dinosaurs’’, modified from the citedsources. Arrows indicate stem-based taxa and black circles node-based taxa. Names applied as in Table 1, not as in the referredpublications. Abbreviations as follows: O, Ornithischia; T, Theropoda; H, Herrerasauridae; E, Eusaurischia.

& Kellner, 2009), which place the North American taxonbasal to the more ‘‘typical’’ herrerasaurids.

Around the turning of the 20th Century, the descriptionof new basal dinosauriforms such as Saturnalia tupiniquim andAgnosphitys cromhallensis, led to the proposal of novel phylo-genetic hypotheses of basal dinosaur relationships (Fig. 9).Langer et al. (1999) described Saturnalia as the basal-mostsauropodomorph, within alternative phylogenetic arrange-ments depicting Herrerasaurus and Staurikosaurus as eithersaurischians basal to the Theropoda+Sauropodomorphadichotomy, or as a monophyletic sister taxon to Dinosauria.Fraser et al. (2002) described Agnosphitys as the sister taxonto Dinosauria, favouring a position of Herrerasaurus outsidethat clade, but these results were not replicated by anyquantitative analyses performed since then.

In a comprehensive analysis of basal theropod phylogeny,Rauhut (2003) recovered Eoraptor lunensis and herrerasauridsas basal theropods, as first proposed by Sereno & Novas(1992). However, most subsequent studies, including somefocused on basal theropods (Yates, 2005; Smith et al., 2007),contradicted that hypothesis of basal dinosaur relation-ships. Yates (2003b) conducted a cladistic study of basalsauropodomorphs, and found a new phylogenetic arrange-ment among saurischians where herrerasaurids were con-sidered the sister group of all other components of theclade, termed Eusaurischia by Padian et al. (1999). Yates(2003b) also found Saturnalia tupiniquim as the most basalsauropodomorph, as previously claimed by Langer et al.(1999) and mainly accepted since. New comprehensive anal-yses by Langer (2004; see also Langer & Benton, 2006) inde-pendently came to similar results (Fig. 9). These corroborated



Fig. 10. Selected anatomical features of Herrerasaurus ischigualastensis, depicting a combination of apomorphic traits shared withNeotheropoda (underlined) and plesiomorphic states relative to the Eusaurischia condition (non underlined). Asterisks indicate traitsalso seen in basal ornithischians according to Butler et al. (2007). Skeletal reconstruction based on Sereno (1993). Scale bar = 10 cm.

the position of herrerasaurids as basal saurischians, addingEoraptor lunensis as the sister taxon to Eusaurischia, andGuaibasaurus candelariensis as a basal theropod. Indeed, sev-eral eusaurischian apomorphies are lacking in herrerasaurids(Fig. 10) and/or Eoraptor, as exemplified by a short caudoven-tral premaxillary process, a nasal that possesses a caudolateralprocess and forms part of the dorsal border of the antor-bital fossa, caudal cervical vertebrae longer than cranialtrunk vertebrae, a large medial-most distal carpal, a stoutmetacarpal I with lateral distal condyle distally expanded,a long metacarpal II relative to metacarpal III, and anexpanded distal end of the ischium (Langer & Benton, 2006).

Subsequent studies broadly agree with the above scenario(Irmis et al., 2007a; Nesbitt & Chatterjee, 2008; Martinez& Alcober, 2009), but differ in minor details (Fig. 9). Ina study focused on the non-dinosaurian affinity of theputative coelophysoid Eucoelophysis baldwini (Sullivan & Lucas,1999), Ezcurra (2006) placed herrerasaurids as non-theropodsaurischians, but Eoraptor as a basal theropod, sister taxonof Neotheropoda. In a study of basal sauropodomorph phy-logeny, Upchurch et al. (2007) placed both Herrerasauridaeand Eoraptor as non-theropod saurischians, but consideredthe former group as the sister taxon of Eusaurischia. Yates(2007a, b) expanded his previous studies, adding Agnosphitys

cromhallensis, Guaibasaurus candelariensis, Chindesaurus bryansmalli,and Eoraptor lunensis to an analysis of basal sauropodomorphs.The former three taxa were found as basal theropods, withGuaibasaurus as the sister taxon of a clade including Chin-

desaurus plus Neotheropoda, and Agnosphitys as the mostbasal theropod. Yet, the theropod affinity of Chindesaurus

was challenged by Irmis et al. (2007a), who supported itsmore traditional relation to Herrerasaurus, both lying basal tothe sauropodomorph/theropod dichotomy, as also suggested

by Langer (2004) and Nesbitt et al. (2009). More recently, thediversity of basal members of the sauropodomorph lineagewas increased by the discovery of Panphagia protos (Martinez& Alcober, 2009) and the undescribed sister-taxon of Satur-

nalia tupiniquim (PVSJ 845; Ezcurra, 2008), both from theIschigualasto Formation, of Argentina. Further, Ezcurra(2008) also included Agnosphitys and Guaibasaurus in thatdinosaur lineage. Indeed, Bonaparte et al. (2007) has alreadyproposed a close relation between Guaibasaurus and Saturnalia,forming Guaibasauridae at the base of Saurischia. However,Langer (2004) suggested the theropod, possibly coelophysoid,affinity of Guaibasaurus (see also Upchurch et al., 2007; Langer,Bittencourt & Schultz, 2007a; Bittencourt, 2008).

Several putative basal dinosaurs were never included innumerical phylogenetic analyses, and their affinities areopen to scrutiny. Langer (2004) offered a comprehensivesummary of these records, but this has to be updated withnew information available since. Remes & Rauhut (2005)reassessed the affinity of Alwalkeria maleriensis, first described asa basal theropod (Chatterjee, 1987; Norman, 1990), but laterregarded as a dinosaur of uncertain (Novas, 1989, 1997a)or eusaurischian (Langer, 2004) affinities. Those authorsfound that the holotype represents a chimera, includingpseudosuchian and possible prolacertiform material, but alsosaurischian specimens. Aliwalia rex Galton, 1985b, on theother hand, previously regarded as a herrerasaurid (Galton,1985b; Paul, 1988) or a dinosaur of dubious affinities (Sues,1990; Galton & Van Heerden, 1998; Langer, 2004) wasshown to represent a junior synonym of Eucnemesaurus fortis,therefore a basal sauropodomorph (Yates, 2007a). Nesbittet al. (2007) recently reviewed the status of the isolatedilium previously assigned to Chindesaurus bryansmalli (Long& Murry, 1995), which constitutes the holotype of Caseosaurus



crosbyensis (Hunt et al., 1998) and considered the materialundiagnostic above Dinosauriformes. Other putative earlydinosaurs such as Saltopus elginensis, Spondylosoma absconditum,and Teyuwasu barberenai, have not been studied recently.Indeed, their uncertain affinities as proposed by Langer(2004) are provisionally accepted here (Table 2).

In conclusion, recent cladistic analyses of basal dinosaurrelationships agree in various aspects, which are accepted bymost of the authors mentioned above: (1) dinosaurs representa monophyletic group exclusive of forms such as Lager-

peton chanarensis, Marasuchus lilloensis, Pseudolagosuchus major,and Silesaurus opolensis; (2) Dinosauria is composed of twomain lineages, Saurischia and Ornithischia; (3) Pisanosaurus

mertii is a basal ornithischian; (4) Herrerasaurus ischigualastensis

and Staurikosaurus pricei belong into a monophyletic Her-rerasauridae; (5) Eoraptor lunensis, Guaibasaurus candelariensis,and herrerasaurids are saurischians; (6) Saurischia includestwo main groups, Theropoda and Sauropodomorpha; and(7) Saturnalia tupiniquim and Panphagia protos are basal membersof the sauropodomorph lineage.

On the contrary, several aspects of basal dinosaur phy-logeny remain controversial. These include the positionof herrerasaurids, Eoraptor lunensis, and Guaibasaurus cande-

lariensis as basal theropods or basal saurischians, and theaffinity and/or validity of various more fragmentary taxasuch as Agnosphitys cromhallensis, Alwalkeria maleriensis, Chinde-

saurus bryansmalli, Saltopus elginensis, Spondylosoma absconditum,and Teyuwasu barberenai. Other equally incomplete formshave been more thoughtfully studied, but while the affinitiesof Aliwalia rex are better understood, Caseosaurus crosbyen-

sis continues to be problematic. In a reappraisal of themethodologies employed in recent analyses of basal dinosaurrelationships, Sereno (2007b) highlighted that the lack ofconsensus regarding the same phylogenetic problematicsis mainly due to differences in character/character-statechoice and codification among authors. Indeed, it seems thatmore comprehensive studies, discussing these methodologi-cal issues, are necessary to achieve a better understanding ofthe phylogenetic relationships of basal dinosaurs. This is, inturn, essential to recognize the patterns leading to their earlyradiation and success during post-Triassic times.

(3) Geographical distribution of basal dinosaurs

The earliest records of dinosauromorphs and dinosauriformsbased on body fossils, and also most of the trustworthyrecords of the earliest dinosaurs come from southern SouthAmerica, especially Argentina (e.g. Bonaparte, 1975; Sereno& Novas, 1992; Sereno & Arcucci, 1993, 1994; Novas,1992b, 1996; Langer et al., 1999; Galton, 2000; Rogers et al.,2001; Langer, 2004; Ferigolo & Langer, 2007; Martinez &Alcober, 2009). However, relatively few sites representativeof terrestrial ecosystems of that time are known (Hammer,Collinson & Ryan, 1990; Lucas, 1998; Rogers et al., 2001;Weishampel et al., 2004) and no biogeographic hypothesisconcerning the area of origin of the dinosaurian cladescan be robustly tested (Parker et al., 2005). Nonetheless,whereas almost all south-Pangean tetrapod-bearing deposits

of Carnian age (Langer, 2005b) bear undisputed, even ifinconspicuous dinosaur records, the north-Pangean scenariois rather different, with no dinosaur positively identified incoeval tetrapod assemblages. Accordingly, an admittedlytentative scenario can be drawn, hinting at a southernPangean origin of dinosaurs.

Obviously, any biogeographical picture of dinosaur originshas to be backed up by the current phylogenetic hypothe-ses depicting the relationships of the basal members ofthe group and its sister taxa. Accordingly, recent finds ofbasal dinosauromorphs in Europe (Fraser et al., 2002; Dzik,2003) and North America (Ezcurra, 2006; Irmis et al., 2007a;Nesbitt & Chatterjee, 2008) came with new phylogeneticproposals, and indicate that those animals had a broadergeographical and chronostratigraphic distribution than pre-viously thought. Hypotheses that support an inclusive cladeof basal dinosauriformes (i.e. Silesauridae) as the sister taxonto Dinosauria (Nesbitt et al., 2007; Brusatte et al., 2008a)face the problem of a Ladinian ghost-lineage of ‘‘stem-dinosaurs’’ (Fig. 3A), but are roughly in agreement withthe southern origin scenario. Spondylosoma absconditum, fromsouth Brazil, could fill that temporal gap, but its atypicalmorphology and uncertain affinity (Langer, 2004) preventfurther scrutiny. The alternative pectinate topology (Ezcurra,2006) overcomes the ghost-lineage problem, but suggests thatnorth Pangean taxa represent the immediate outgroups toDinosauria (Fig. 3A), jeopardizing the ‘‘out of south Pangea’’model of dinosauromorph/dinosauriform/dinosaur radi-ation. The record of Ladinian-Carnian ‘‘dinosauri-form/dinosaur’’ footprints in various parts of the world (Mel-chor & De Valais, 2006; Thulborn, 2006; Marsicano et al.,2007) also hints at a broader distribution of these basal forms.

Late Triassic dinosaur records as a whole include bodyfossils from Europe, North and South America, India,Africa, and East Asia (Weishampel et al., 2004), as wellas putative tracks from Australia (Thulborn, 2000, 2006).This is congruent with the geographic configuration of thetime (Fig. 7), when the Pangea Supercontinent and the lackof extensive oceanic barriers would favour biotic expansion(Shubin & Sues, 1991). Indeed, several non-dinosaur tetrapodclades also achieved a widespread distribution during theLate Triassic and Early Jurassic (Benton, 1993), but itis important to note that no dinosaur clade had a trulyglobal distribution during Late Triassic times, especiallyin the Carnian Stage (Nesbitt et al., 2007), even if onlythe areas with tetrapod-bearing sites are considered. Thebiogeographic patterns of early dinosaur radiation are, infact, better analyzed having the proposed subdivisions of thegroup as a template.

The osteological record of Triassic ornithischians (Fig. 7)is restricted to three Norian forms: the South African Eocursor

parvus (Butler et al., 2007), an unnamed heterodontosauridfrom Patagonia (Baez & Marsicano, 2001), and Pisanosaurus

mertii, from northwestern Argentina (Casamiquela, 1967;Bonaparte, 1976), the latter of which may come from signifi-cantly older deposits. Various other remains, mostly isolated



teeth, from either Europe (Godefroit & Cuny, 1997; Gode-froit & Knoll, 2003) or North America (Chatterjee, 1984;Hunt, 1989; Hunt & Lucas, 1994; Heckert, 2002, 2004)had been assigned to the group. Along with footprints fromNorth America, Europe, and southern Africa, these mayhint at a broader Norian-Rhaetian geographical distribu-tion of ornithischians. Yet, neither the isolated teeth northe footprints can be unequivocally assigned to the group(Parker et al., 2005; Butler et al., 2006; Irmis et al., 2007b).Accordingly, as discussed by Irmis et al. (2007b), ornithis-chians do not seem to have been very diverse or abundantthrough the Triassic, and certain hypotheses of relationship(Sereno, 1991b, 1999; Xu et al., 2006) imply large gaps intheir fossil record. On the contrary, the usually acceptedbasal position of Pisanosaurus is in accordance with its olderage, as is the possible basal position of heterodontosaurids(Butler, Upchurch & Norman, 2008) and Eocursor (Butleret al., 2007) in relation to other ornithischians. The suggestedheterodontosaurid affinity of Pisanosaurus (Bonaparte, 1976;Galton, 1986; Crompton & Attridge, 1986; Butler et al., 2008)implies a minimal ghost-lineage for Genasauria sensu Butleret al. (2008), but is also in general agreement with a southPangean origin of ornithischians. Indeed, Laurasian occur-rences of the group can not be confirmed before the EarlyJurassic (Irmis et al., 2007b), when basal thyreophorans occurin North America, Europe, and Asia (Norman, Witmer &Weishampel, 2004a; Irmis & Knoll, 2008). A different pictureemerges with the tentative placement of Sacisaurus agudoensis

and especially Silesaurus opolensis as the most basal ornithis-chians (Ferigolo & Langer, 2007), but this hypothesis is stillto be backed up by numerical phylogenetic analyses. In anycase, the poorly documented early history of ornithischiansprevents any accurate biogeographic approach. Accordingto Irmis et al. (2007b), possible explanations for their rarityin Late Triassic rocks (e.g. sample bias, differential environ-mental occupation, systematic imprecision) are inconclusive.

Triassic saurischians have a much broader geographicdistribution (Rauhut & Hungerbuhler, 2000; Langer, 2004;Nesbitt et al., 2007). Indeed, basal members of the group,and putative members of the theropod and sauropodomorphlineages occur as body fossils in various Carnian bedsknown from south Pangea (South America, southern Africa,and India) as well as in Norian-Rhaetian deposits ofall continents except Australia and Antarctica (Fig. 7).However, most records of ‘‘basal saurischians’’ are, infact, records of saurischians of uncertain affinities, andonly Eoraptor lunensis and herrerasaurs have been, undercertain phylogenetic hypotheses, positively placed basal toEusaurischia. Well-known herrerasaurids are restricted tothe South American Carnian (Langer, 2004; Bittencourt& Kellner, 2009), including Herrerasaurus ischigualastensis andStaurikosaurus pricei. The clade remains unidentified in therelatively well-known post-Ischigualastian deposits of thatcontinent, hinting at its restricted stratigraphic distribution.Yet, herrerasaurids have been identified in Norian bedsof western USA (Long & Murry, 1995; Hunt et al., 1998;Irmis et al., 2007a; Nesbitt & Chatterjee, 2008), which would

represent the younger records of the group. In addition,given that the affinity of Chindesaurus bryansmalli to eitherof the South American herrerasaurids has been questioned(Langer, 2004; Bittencourt & Kellner, 2009), that NorthAmerican herrerasaur would most probably represent theremnant of a lineage parallel to the typical members of thegroup, and not a more derived outcome of that radiation.On the contrary, the latter seems to be the case for thespecimen described by Nesbitt & Chatterjee (2008), whichbears herrerasaurid apomorphies.

If herrerasaurids and/or Eoraptor lunensis are treated astheropods, the group would fit the ‘‘out of South Pangea’’radiation pattern, with a well-known record of basal formsin the Carnian of South America. In the alternativearrangement (Langer, 2004), the oldest theropod, i.e. thecoelophysoid ‘‘Camposaurus arizonensis’’ (Nesbitt et al., 2007),would not only come from North America, but also fromNorian-age deposits. If not filled by herrerasaurids and/orEoraptor this stratigraphic gap in theropod distribution isunexpected, given the occurrence of basal members ofthe sauropodomorph lineage in the Carnian (Langer et al.,1999; Ezcurra, 2008; Martinez & Alcober, 2009), andthe abundance of both saurischian groups later in theTriassic (Tykoski & Rowe, 2004; Galton & Upchurch, 2004).Indeed, mainly represented by coelophysoids (but see Nesbitt& Chatterjee, 2008), theropods become abundant duringNorian-Rhaetian times (Fig. 7), with body fossils recordedin North America (Jenkins et al., 1994; Nesbitt et al., 2007),Europe (Rauhut & Hungerbuhler, 2000; Ezcurra & Cuny,2007), Argentina (Arcucci & Coria, 2003; Ezcurra & Novas,2007a), India (F. E. Novas, personal observations), andperhaps South Africa (Ray & Chimsamy, 2002). The possibletheropod affinity (Yates, 2007a, b) of controversial Noriantaxa from Europe (Agnosphitys cromhallensis), North America(Chindesaurus bryansmalli), Brazil (Guaibasaurus candelariensis),and India (aff. Guaibasaurus) does not significantly change thisdistribution pattern. In comparison to the more abundantsauropodomorphs, osteological records of theropods arelacking in Norian-Rhaetian deposits from southwest Asia(Nam Phong Formation), suggesting that Pangean far-eastwas first reached by the herbivorous/omnivorous branch ofSaurischia. Yet, this fossil assemblage is imperfectly known(Buffetaut et al., 2000), and the absence of theropods mightsimply represent a circumstantial sample bias. The scarcity oftheropod body fossils of Late Triassic age in southern Africa issomewhat filled by ichnological evidence (Ellenberger, 1974;Olsen & Galton, 1984; Raath et al., 1990), and also inferredfrom their well-known Early Jurassic record (Raath, 1969;Bristowe & Raath, 2004; Yates, 2005). Possible theropodfootprints are also known form Norian-Rhaetian deposits ofother parts of the world (Gatesy et al., 1999; Haubold & Klein,2002; Demathieu & Demathieu, 2004; Thulborn, 2006), butseveral of them have been questioned (King & Benton,1996; Marsicano et al., 2007; Lucas, 2007; Nesbitt et al.,2007), given their possible assignment to non-dinosauriandinosauromorphs. In any case, the overall record leads to ascenario of low abundance of Carnian theropods, followed



by a significant Norian radiation, when the group occupiedmost parts of Pangea.

Sauropodomorphs are surely the most abundant dinosaurgroup of Triassic times. Basal members of the lineage, i.e.Saturnalia tupiniquim and its allies, come from Ischigualastianbeds of South America (Langer et al., 1999; Da Rosa et al.,2006; Ezcurra, 2008; Martinez & Alcober, 2009), and pos-sibly southern Africa (Raath, 1996). This record could beenhanced by the ‘‘prosauropods’’ of the Lower Maleri For-mation referred to by Kutty & Sengupta (1989), but thesehave not been mentioned in more recent studies (Kutty et al.,2007), which refer the Upper Maleri ‘‘prosauropods’’ to aff.Guaibasaurus. Assuming the above identifications as correct,a southern radiation of ‘‘Saturnalia-like’’ forms may have pre-ceded the Norian diversification of true sauropodomorphs.The basal-most members of that group, Pantydraco caducus

and Thecodontosaurus antiquus (Yates, 2007a, b), come fromfissure-filling deposits of England and Wales of alleged Car-nian age, but this occurrence better fits the much broaderdistribution of later sauropodomorphs (Fig. 7). Indeed, theNorian-Rhaetian record of the group excludes only Antarc-tica, Australia, and continental North America (Vickers-Richet al., 1999; Galton & Upchurch, 2004; Nesbitt et al., 2007).The former two areas, however, lack well-sampled tetrapodfaunas of that age, and the absence of sauropodomorphscould represent a sampling bias. Indeed, the group can besaid to have had a nearly global Norian distribution, but thiswas not uniform through time and space. Most of the basal,non-Plateosauria (sensu Yates, 2007a) taxa were recordedin Europe (Yates, 2003b,c; Galton & Upchurch, 2004;Galton, 2007), while more derived forms are widespread(Yates, 2007a, b; Galton & Upchurch, 2004; Leal et al.,2004; Pol & Powell, 2007a). In this context, the lack ofsauropodomorphs in the Norian of continental North Amer-ica (Nesbitt et al., 2007), though not in Greenland (Jenkinset al., 1994), is intriguing. Indeed, this seems to represent atrue biogeographic pattern, given the abundance of well-sampled tetrapod-bearing deposits of that age in the region.The occurrence of the latest basal dinosauromorphs (Irmiset al., 2007a) and probable herrerasaurs (Nesbitt & Chatter-jee, 2008) in those faunas may also result from the causes thatdrove this pattern. As for basal sauropods, previous studiessuggested that their distribution was initially restricted tosoutheastern Asia, expanding throughout Pangea by theLate Triassic-Early Jurassic (Gillette, 2003). Yet, recent phy-logenetic hypotheses (Yates, 2007a, b; Smith & Pol, 2007;Upchurch et al., 2007) have positioned south Pangean formslike Antetonitrus ingenipes, Blikanasaurus cromptoni, Lessemsaurus

sauropoides, and Melanorosaurus readi, close to, or at the baseof Sauropoda. This suggests a wider distribution of earlymembers of the group, a pattern that seems to fit bet-ter the footprint record (Wilson, 2005). In any case, mostTriassic ichnological evidence of sauropodomorphs was con-sidered poorly substantiated (Lockley et al., 1994; Rainforth,2002, 2003).

IV. ECOLOGY OF THE DINOSAUR RADIATION

(1) The Triassic scene

During the Triassic, following the Late Permian maximumcoalescence of Pangea, most continental areas remainedforming a single landmass (Scotese, 2002; Golonka, 2002;Blakey, 2006). Towards the end of the period, major rift zonesstarted to develop, especially along the Atlantic marginsof North America and North Africa, accounting for theseparation between Laurasia and Gondwana (LeTourneau& Olsen, 2003; Golonka, 2007). Besides, the climateexperienced a trend towards higher instability comparedto Paleozoic settings (Holser & Magaritz, 1987; Kent &Muttoni, 2003). The Triassic palaeoclimate was reviewedin various landmark publications (Tucker & Benton, 1982;Hallam, 1985; Parrish, 1993; Crowley, 1994; Golonka &Ford, 2000), which suggest a warm period, when polar icecaps were absent (Frakes, Francis & Syktus, 1992). Further,a latitudinal zonation seems to have been present, with anarid equatorial/tropical belt, a seasonally humid temperatezone, and mainly humid higher latitudes (Hallam, 1985; butsee Fraser, 2006). In the second half of the period, a highlyseasonal (monsoonal) humid climate prevailed over variousparts of the supercontinent (Parrish, 1993). Triassic biotasreflect the transitional nature of the time interval (Anderson &Anderson, 1993; Fraser, 2006), particularly when terrestrialtetrapod faunas are considered. The period starts with theimpoverished remaining diversity of the end-Permian massextinction (Benton, 2003), ending up with an essentiallymodern fauna, that includes the first representatives of thechelonian, lepidosaur, crocodilian, avian (in the form ofdinosaurs), and mammal lineages.

Part of the Triassic tetrapod diversity was inheritedfrom the Permian, when dicynodonts and limnarchiantemnospondyls reached their climax (King, 1988; Milner,1993). These groups experienced a later diversification withinthe Triassic, along with the first radiation of lineages of latestPermian origin, like procolophonoids (Spencer & Benton,2000), ‘‘protorosaurs’’ (Dilkes, 1998), archosaurs (Gower &Sennikov, 2000), and cynodonts (Botha, Abdala & Smith,2007). These tetrapod groups diversified through the EarlyTriassic, composing the core of the Middle Triassic pre-dinosaur terrestrial palaeocomunities; the ‘‘Kannemeyeroidepoch’’ of Ochev & Shishkin (1989). An example of suchfaunas is known from the Chanares Formation, Argentina(Bonaparte, 1982; Rogers et al., 2001) that is dominatedby herbivorous cynodonts (Massetognathus) and dicynodonts(Dinodontosaurus), along with predatory cynodonts (Chiniquodon)and archosaurs (proterochampsids and ‘‘rauisuchians’’).Nearly coeval faunas were recorded in Brazil, Russia, andSouthern Africa, further including a variety of limnarchians,procolophonids, and rhynchosaurs (Ochev & Shishkin, 1989;Lucas, 1998; Abdala & Ribeiro, 2003). In addition, aspreviously discussed, the Chanares fauna also includes thehighest diversity of basal dinosauromorphs (Romer, 1971,1972a, b; Bonaparte, 1975; Sereno & Arcucci, 1993, 1994).



The oldest known dinosaurs are recorded in a particularfaunal context, in which rhynchosaurs, especially thegenus Hyperodapedon, became dominant primary consumersof various terrestrial faunas worldwide (Romer, 1962;Benton, 1983b). These were recorded in the Hyperodapedon

Assemblage-Zone of the Santa Maria Formation (Fig. 11A),the lower part of the Ischigualasto Formation, theLossiemouth Sandstone Formation, and the Lower MaleriFormation (Langer, 2005b). Apart from rhynchosaurs andthe first dinosaurs, these faunas collectively encompasslimnarchian temnospondyls (Marsicano, 1999; Sengupta,2003), various archosaurs such as proterochampsids (Price,1946; Sill, 1967), ‘‘rauisuchians’’ (Huene, 1942; Alcober,2000), poposauroids (Alcober & Parrish, 1997), aetosaurs(Heckert & Lucas, 2002), phytosaurs (Chatterjee, 1978),crocodylomorphs (Bonaparte, 1982; Ezcurra, Lecuona &Irmis, 2008), and ornithosuchids (Benton & Walker, 1985),carnivorous (Martinez, May & Forster, 1996; Bonaparte& Barberena, 2001; Abadala & Gianinni, 2002) andherbivorous (Bonaparte, 1962; Chatterjee, 1982; Hopson,1985) cynodonts; as well as dicynodonts (Cox, 1965).A similar faunal content was recorded in putatively coevalfaunas that lack rhynchosaurs and dinosaurs such as thatof Krasiejow, Poland (Dzik & Sulej, 2007), and the base ofthe Irohalene Member (Timesgadiouine Formation, ArganaBasin), Morocco (Jalil, 1996). In fact, apart from theappearance of some archosaur groups, and the dominance ofrhynchosaurs, the oldest dinosaur-bearing terrestrial faunasare not significantly different from those of Middle Triassicage.

Dinosaurs remain inconspicuous in younger faunas ofNorian age, as seen at the base of the Los ColoradosFormation (Caselli, Marsicano & Arcucci, 2001) and theCaturrita Formation (Langer et al., 2007b), in South America,and in some possibly coeval North American fossil assem-blages (Langer, 2005b), i.e. Sanfordian faunas of the NewarkSupergroup; Camp Springs Member, Dockum Group; andPopo Agie Formation (Huber, Lucas & Hunt, 1993; Lucas,1998). These faunas include metoposaurids, procolophonids,sphenodontians, Hyperodapedon, dicynodonts, traversodontidand mammal-like cynodonts, as well as various pseudo-suchians (aetosaurs, phytosaurs, poposauroids, and possible‘‘rauisuchids’’). Later Norian deposits include a greater num-ber of dinosaur records, within a slightly dissimilar faunalcontext; the ‘‘Prosauropod’’ Empire of Benton (1983a).As recorded from the top of the Los Colorados Forma-tion, the fauna of La Esquina (Bonaparte, 1982; Caselliet al., 2001) includes some of the oldest turtles, alongwith crocodyliforms, ‘‘remaining’’ pseudosuchian lineages(aetosaurs, ornithosuchids, ‘‘rauisuchians’’), and mammal-like cynodonts (Fig. 11B). Putatively coeval faunas of otherparts of the world, especially South Africa (Anderson, Ander-son & Cruickshank, 1998), Europe (Benton, 1994a), andNorth America (Long & Murry, 1995), further includevarious temnospondyls and phytosaurs, inconspicuous dicyn-odonts (Dzik et al., 2008), the latest traversodontids (Hopson,1984), as well as the first mammals (Lucas & Luo, 1993).

(2) Lucky break?

Palaeoecological aspects of the early radiation of dinosaursand its correlation to Late Triassic extinctions andcorresponding biotic/environmental changes have beenaddressed by various classical and more recent studies(Colbert, 1958; Benton, 1983a; Charig, 1984; Olsen et al.,2002; Tanner, Lucas & Chapman, 2004; Brusatte et al.,2008a, b). Focus has been given to two inferred massextinction events, at the Carnian-Norian and Triassic-Jurassic boundaries, and two alternative scenarios for therise of the dinosaurs, the so-called ‘‘competitive’’ and‘‘opportunistic’’ models. Studies from the mid-late 20thCentury postulated that the replacement of various tetrapodgroups, notably pseudosuchians and therapsids, by dinosaurswas a long-term affair driven by competition during theLate Triassic (Cox, 1967; Charig, 1980; Bonaparte, 1982).Its outcome would have been the dominance of dinosaursover terrestrial ecosystems from Norian/Jurassic onwards,thanks to their ‘‘superiority’’ relative to the outcompetedcontemporary tetrapods, pushed to extinction. Usually, theimproved locomotory capability of the fully erect, bipedalearly dinosaurs was considered the most notable advantage ofthe group (Charig, 1972, 1984), but their inferred advancedphysiology has also been mentioned (Bakker, 1971). Fromthe 1980s onwards, Benton (1983a, 1984, 1991) advocatedan alternative model, based on which the Triassic radiationof dinosaurs was faster, opportunistically occupying adaptivezones emptied by the extinction of rhynchosaurs, therapsids(dicynodonts and some cynodonts), and pseudosuchians(phytosaurs, aetosaurs, rauisuchians). More recently, Brusatteet al. (2008a) demonstrated that Norian pseudosuchiansoccupyied more morphospace and showed similar ratesof character evolution compared to dinosauromorphs/dinosaurs. Indeed, this dismisses the classical ‘‘competitive’’model, based on which those archosaurs were graduallyreplaced by dinosaurs. Yet, the scenario seems to be morecomplex in terms of patterns and timing of biotic turnovers,as discussed below.

Of the two proposed Late Triassic extinction events(Benton, 1986a, 1997), the end-Triassic is much betterdocumented in the literature than the end-Carnian, whichis often contested as minor or non-existent (Olsen & Sues,1986; Olsen, Shubin & Anders, 1987; Hallam, 1990; Fraser& Sues, 1994; Hunt et al., 2002). Classical studies revealthe final demise of conodonts and a severe reductionin the diversity of sponges, scleractinian corals, molluscs(ammonoids, gastropods, and bivalves), and brachiopods inthe sea (Hallam, 1981; Raup & Sepkoski, 1982; Sepkoski,1982, 1990), along with extinctions of insects (Benton, 1989)and tetrapods on land (Olsen & Sues, 1986; Benton, 1994b).Causes proposed for the end-Triassic mass extinction rangefrom sea level change (Hallam, 1990), to the impact ofone or more extraterrestrial bolides (Olsen et al., 2002)and the establishment of the Central Atlantic MagmaticProvince (Marzoli et al., 1999). The latter two might haveled to an increase in the levels of atmospheric CO2, and so‘‘greenhouse’’ warming (McElwain, Beerling & Woodward,



Fig. 11. Reconstruction of two dinosaur-bearing fossil assemblages of the South American Late Triassic. (A) Alemoa fauna (SantaMaria Formation), Carnian of south Brazil, depicting from left to right the aetosaur Aetosauroides sp.; the rhynchosaur Hyperodapedonmariensis; the stem-sauropodomorph Saturnalia tupiniquim (group on background); the cynodont Prozoostrodon brasiliensis (in front), andthe herrerasaurid Staurikosaurus pricei. (B) La Esquina fauna (Los Colorados Formation), Norian of northwestern Argentina, depictingon the left (from back to front), a group of the sauropodomorph Riojasaurus incertus; the ‘‘rauisuchid’’ Fasolasuchus tenax, and thecynodont Chaliminia musteloides; on the right (from back to front), the crocodyliform Hemiprotosuchus leali, and the basal theropodZupaysaurus rougieri subduing the ornithosuchid Riojasuchus tenuiceps. Drawings by Jorge Blanco.



Fig. 12. Distribution of medium to large sized terrestrial amniotes along the Late Triassic and Early Jurassic and the rise of dinosaurs.Timeline from Gallet et al. (2003). Distribution of carnivorous (grey columns) and herbivorous/omnivorous (black columns) tetrapodsmodified from Benton (1994a), according to Dilkes (1998), Abdala & Giannini (2002), Abdala & Ribeiro (2003), Thulborn & Turner(2003), Lucas & Tanner (2005), Langer et al. (2007c), and Brusatte et al. (2008a). 1, Saturnalia tupiniquim; 2, Herrerasaurus ischigualastensis;3, Guaibasaurus candelariensis; 4, Eocursor parvus; 5, Plateosaurus engelhardti; 6, Liliensternus liliensterni; 7, Massospondylus carinatus; 8, Vulcanodonkaribaensis; 9, Heterodontosaurus tucki; 10, Dilophosaurus wetherilli; 11, Scelidosaurus harrisoni. Silhouettes (roughly at the same scale) adaptedfrom various sources. Mys, million years before recent; Rhaet., Rhaetian.

1999). Yet, Tanner et al. (2004; see also Bambach, Knoll &Wang, 2004) compiled evidence to reject what they call ‘‘themyth of a catastrophic extinction at the Triassic-Jurassicboundary’’. Indeed, as already hinted by some (Benton,1994b; Cuny, 1995; Ezcurra & Cuny, 2007), although variouswell-known Triassic amniote groups have no Jurassic record,some might have gone extinct before the Triassic upperboundary (Fig. 12).

The pioneering studies of Benton (1983a, 1986a, 1989,1994b), which first challenged the long-term ‘‘competitive’’model of dinosaur radiation, also advocated that the mainturnover of terrestrial faunas occurred at the Carnian-Norian, rather than at the Triassic-Jurassic boundary.This would have been characterized by the extinction ofvarious tetrapod groups, connected to climatic/floral changes(Tucker & Benton, 1982). Problems related to that modelinclude: (1) new data suggest the survival, at least untilthe initial stages of the Norian, of taxa believed to havegone extinct at the end of the Carnian (Fig. 12); (2) lackof synchronicity between the climatic/ floral changes ofnorth and south Pangea. Indeed, dicynodonts occur inNorian faunas of South (Langer et al., 2007c) and NorthAmerica (Long & Murry, 1995), in the latest Triassic ofPoland (Dzik et al., 2008), and perhaps in much younger

assemblages as well (Thulborn & Turner, 2003). In addition,the Caturrita Formation, of south Brazil, has yielded thelatest (Norian) remains of proterochampsid archosaurs andrhynchosaurs (Langer et al., 2007c), while lagerpetonidsand herrerasaurids were recorded in the Norian of USA(Irmis et al., 2007a; Nesbitt & Chatterjee, 2008; Nesbittet al., 2009). The diversity of chiniquodontid cynodonts, onthe other hand, has been reduced to a single genus ofAnisian-Carnian distribution in South America and southernAfrica (Abdala & Giannini, 2002; Abdala & Smith, 2009).Accordingly, its absence in Norian strata does not representthe demise of a well-established lineage, as is also thecase of single-genus ‘‘families’’ such as Pisanosauridae andScleromochlidae (Benton, 1994b). On the other hand, itis important to stress that rhynchosaurs and dicynodontsget less common in Triassic faunas after the Carnian. Thisabundance shift, rather than their extinction, could indeedprovide some evidence for a biological crisis at the Carnian-Norian boundary. In the marine realm, classical studiessuggested that invertebrate extinctions were not conspicuousat the end of the Carnian (Simms & Ruffell, 1990), onlyfew groups suffering a moderate loss of diversity (Schafer& Fois, 1987; Smith, 1988). Yet, more recent data onthe so-called Reingraben Turnover suggest that a major



restructuring of marine ecosystems occurred at the Carnian-Norian boundary (Hornung & Brandner, 2005; Stanley,2006; Hornung, Krystyn & Brandner, 2007b).

Despite Cornet & Olsen’s (1990) statement againstsignificant floristic changes across the Carnian-Norianboundary, this is assumed in a climate-driven fashion by thebiotic substitution model of Benton (1983a). Most authorsagree on the occurrence of a monsoonal humid phase(Fig. 12), minimally affecting the northern Tethyan realmduring the mid-late Carnian (Simms, Ruffel & Johnson,1994; Hornung et al., 2007a). Yet, although a humid phasewas also recorded in the Late Triassic of other parts ofthe world, it seems to have lasted until later within theperiod in those areas. Prochnow et al. (2006) suggest thatapproximately during the deposition of the Petrified ForestMember of the Chinle Formation, presently considered ofNorian age (Nesbitt et al., 2007), western North Americawas experiencing a period of increasing humidity. Similarly,the Santa Maria Supersequence, in south Brazil, showsthe progressive replacement of an ephemeral anastomosedfluvial-lacustrine system by a perennial braided fluvial systemthroughout the Carnian and Norian (Holz & Scherer, 2000;Zerfass et al., 2003), whereas the Norian upper third of theIschigualasto Formation bears evidence of a humidity peak,based on plant taphonomy and the occurrence of argiliticpalaeosols (Colombi & Parrish, 2008). Besides, Dicroidium-floras occur through the entire sequence, as is also the casein other parts of south Pangea (White, 1990; Anderson &Anderson, 1993), while bennettitaleans and conifers onlydominate after the Norian (Fig. 12).

What seems to occur during the Norian is a rise in theabundance of dinosaurs (Fig. 12). These are inconspicuousin Carnian faunas, representing about 5% of tetrapodfossils collected in the Hyperodapedon Assemblage-Zone of theSanta Maria Formation (Azevedo, Schultz & Barberena,1990; Langer et al., 2007c) and the lower third of theIschigualasto Formation (Bonaparte, 1982; Rogers et al.,1993). Carnivorous dinosaurs were proportionally moreabundant, representing nearly 40% of all terrestrial meat-eaters, and half of the medium- to large-sized predatorsof the latter assemblage (Rogers et al., 1993). By contrast,dinosaurs represent from 25 to 60% of the terrestrialtetrapods of classical Norian faunas (Benton, 1983a), notablyin South Africa (Kitching & Raath, 1984), Argentina(Bonaparte, 1982), and Europe (Benton, 1994a). This is betterdocumented for saurischians, especially with the notableradiation of ‘‘prosauropods’’. However, dinosaurs are stillminor components of possibly older Norian faunas such asthat of the Caturrita Formation (Langer et al., 2007c), wherethey represent about 15% of the known diversity. Althoughthe total number of dinosaurs registered in the Norian is atleast three times higher than in the Carnian, Ezcurra & Novas(2008) emphasize that dinosaur diversity in the IschigualastoFormation surpasses that of most Norian stratigraphic units(Fig. 7). This may imply that their radiation in the latterstage resulted in a more abundant and disparate (Brusatteet al., 2008b), but not necessarily more diverse, dinosaur

fauna, leaving the greater total diversity of Norian dinosaursto be explained based on the existence of more tetrapod-bearing sites of that age. Yet, various main dinosaur groupsappear to have originated and/or radiated during the Norian,as is the case for heterodontosaurids, coelophysoids, and‘‘prosauropods’’.

Moreover, as discussed by Novas (1997b; contra Benton,1983a), the Late Triassic dinosaur rise did not occur inan empty ecospace. Instead, dinosaurs radiated during theIschigualastian despite the high diversity and abundanceof other tetrapods. Especially in the case of carnivores,it is difficult to deny a degree of overlap in the use offood resources by sympatric species of similar size (Glen &Dickman, 2008); whereas modern analogues suggest thatfeeding niche overlap is less significant among herbivores(Plumptre, 1996; Begon, Harper & Townsend, 1996, p. 778).In this context, it is possible to envisage strictly herbivorousTriassic dinosaurs, which were not many (see SectionIV.3), using plant resources not previously exploited infull. Yet, counterevidence is given by the non-overlap ofthe rhynchosaur bearing Norian assemblages with youngerfaunas of that age (Fig. 12), in which dinosaurs are moreabundant (Langer, 2005a). In this context, the Norianradiation of herbivorous dinosaurs could be linked toecological release, given the extinction of rhynchosaurs (seealso Brusatte et al., 2008b). The corollary, as suggestedby Novas (1997b), is that competitive pressure of non-dinosaur herbivores may account for the low abundanceof herbivorous dinosaurs in the Carnian.

Whatever the physical causes of the end-Triassic event,direct ‘‘extermination’’ is unlikely to have been the onlyagent of the extinctions. Instead, the exacerbation ofbiotic interactions (including competition) in a changingenvironment probably also played a major role. In thatcontext, the already abundant and diverse dinosaurs mayhave had the key adaptations to succeed and expand,within the shifting environment that drove various othertetrapods towards extinction. This is the second explanationoffered by Brusatte et al. (2008a) accounting for the extinctionof pseudosuchians, and not dinosaurs, at the Triassic-Jurassic boundary. It implies a circumstantial ‘‘superiority’’of dinosaurs, while historical burden enforces the reference toa fully erect gait (Charig, 1972) and/or advanced physiology(Bakker, 1971; Ward, 2006) within the set of dinosaur‘‘advantages’’. This explanation is preferred here, given thatthe first is based on ‘‘chance’’ (Brusatte et al., 2008a), whichjust reflects cases ‘‘when our knowledge does not sufficefor prediction’’ (Popper, 1959). In fact, as acknowledgedby Popper (1959): we may not infer from the fact that anevent is chance-like that its elements are ‘due to chance’.This is because ‘‘chance’’ and lack of knowledge producethe same signatures, and can not be set apart in practice. Inour understanding, opportunistic and competitive scenariosof dinosaurs rise are not mutually exclusive, and competitionmay have played an important role in that radiation episode.Not as a long-term affair, but triggered by the physical eventsultimately linked to the end-Triassic extinction.



Although the Early Jurassic total diversity of dinosaurgenera is not significantly higher than the Late Triassic(Wang & Dodson, 2006), it is usually accepted that anotherpulse of dinosaur radiation followed the Triassic-Jurassicextinction (Olsen et al., 2002). This is better measured by theorigin of certain lineages and exploitation of new ecologicalroles (Fig. 12). Among saurischians, large carnivores (Ezcurra& Novas, 2007a) and large quadruped herbivores (Yates &Kitching, 2003) were already known in the latest Triassic.In fact, the record of Zupaysaurus (Ezcurra & Novas, 2007a),and studies of Brusatte et al. (2008b), falsifies the hypothesisof theropod size increase due to ecological release afterthe Triassic-Jurassic boundary (Olsen et al., 2002; Lucas &Tanner, 2005). Yet the Early Jurassic saw the radiationof the Dilophosaurus-clade (Smith et al., 2007) and furtheracquisition of typical graviportal traits among sauropods(Barrett & Upchurch, 2007). The radiation of ornithischianswas more notable, with heterodontosaurids attaining theirdiversity peak and the origin of both neornithischians andthyreophorans (Butler et al., 2007). The latter representsthe debut of quadrupedal armoured forms, within amorphospace previously unoccupied by dinosaurs.

In conclusion, the radiation of dinosaurs comprises at leastthree landmark moments (Fig. 12), mainly characterizedby early diversification (Carnian); increase in diversityand, especially, abundance (Norian); and occupation ofnew niches (Early Jurassic). As previously mentioned, theCarnian diversification did not occur in an empty ecospace,but despite the abundance and diversity of contemporarytetrapods. The Norian increase in dominance might beconnected to climatic/ floristic changes and to the extinctionof herbivorous forms such as rhynchosaurs, but the timing ofthese events needs further investigation. The subtle Jurassicdiversification, on the other hand, seems to have occurred inthe aftermath of an extinction event (Brusatte et al., 2008a).Indeed, this might be an example of opportunistic radiationinto released ecospace (Benton, 1983a; Olsen et al., 2002;but see Brusatte et al., 2008b). Obviously, this does not fitthe notion that the ‘‘end-Triassic’’ tetrapod extinctions werescattered over the end of the period (Tanner et al., 2004).Indeed, the lack of various tetrapod groups in Rhaetian bedsand the less than expected dinosaur diversity increase in theEarly Jurassic seem to justify this latter scenario, but the‘‘diversity loss’’ of dinosaurs and other tetrapods during theRhaetian is likely to be due to sampling bias (Ezcurra &Cuny, 2007). In any case, post-Triassic tetrapod biodiversitycan not be understood as the outcome of a single event, butseems modeled by long-term coexistence of different groupsduring the Late Triassic. Punctual events of environmentalchange may have enhanced interaction among lineages,leading to the extinction of some terrestrial forms. This wasprobably topped by a final historical contingency at theTriassic-Jurassic boundary, when dinosaur circumstantial‘‘superiority’’ set the frame for the next 135 million years ofarchosaur evolution.

(3) Of legs and teeth: insights on the palaeobiologyof early dinosaurs

Non-crown-group archosaurs were all quadrupedal andcarnivorans (Charig, 1972; Parrish, 1986; Brusatte et al.,2008a). This general pattern was retained in basal membersof the crocodile-line (Bonaparte, 1984; Sereno, 1991a),although recent discoveries of bipedal (Nesbitt, 2007) andherbivorous/omnivorous (Parker et al., 2005) pseudosuchiansnotably amplified the disparity of these archosaurs. Thebasalmost dinosaurs, instead, were all bipedal, but it is notclear if this was also the case for more basal dinosauromorphs,and several instances of reversion to full or facultativequadrupedalism are known within the group (Padian, 1997c).The feeding habit of basal dinosaurs is even more difficult toassess. Basal dinosauromorphs have a rather unspecializeddentition, but typical carnivores and herbivores occur earlyin dinosaur evolution (Barrett, 2000).

As seen in the previous section, bipedalism is often consid-ered a key dinosaur feature that, along with a fully erect pos-ture, favoured the radiation of the group during Late Triassictimes. Yet, supporters of a polyphyletic origin of dinosaurssuggested that some quadrupedal lineages, particularlysauropods (Charig et al., 1965), have never had bipedal ances-tors, evolving directly from quadrupedal basal archosaurs(Fig. 2). The general acceptance of dinosaur monophyly, andthe identification of its sister taxa within gracile, and presum-ably bipedal Middle Triassic archosaurs (Gauthier, 1986) set-tled the new orthodoxy of originally bipedal dinosaurs, whichwas in turn challenged by more recent data. This includes thediscovery of a potentially quadrupedal basal dinosauromorph(Dzik, 2003), and new ichnological (Haubold & Klein, 2002)and biomechanical (Fechner, 2006) interpretations that hintat higher degrees of quadrupedalism among dinosaur precur-sors. It is consensual that herrerasaurs, basal theropods, andbasal ornithischians were fully bipedal (Carrano, 2000; But-ler et al., 2007), but the condition among sauropodomorphsis less clear (Cooper, 1981; Barrett & Upchurch, 2007). Inany case, only if all evidence in favour of basal dinosauro-morph/dinosaur quadrupedalism is accepted, and optimizedon a favourable phylogenetic framework (Fig. 13) does aquadrupedal/facultative bipedal origin of dinosaurs emergeas parsimoniously as a fully bipedal origin. Otherwise, the lat-ter hypothesis is always favoured if herrerasaurs are regardedas basal saurischians (Yates, 2003b; Langer, 2004; Ezcurra,2006; Irmis et al., 2007a). In fact, a fully quadrupedal dinosaurorigin is consistently ruled out, given that basal members ofthe sauropodomorph lineage, even if capable of walking onall fours, must have relied on bipedalism for higher speedlocomotion (Christian & Preuschoft, 1996; Upchurch, 1997a;Langer, 2003). Actually, this may be the case also of Silesaurus

opolensis, the unusual slender fore limbs of which may nothave endured the same amount of stress as the hind limbs did(see Farina, 1995). Evidently, more detailed biomechanicalstudies of basal dinosauromorphs are needed to recognize ifdinosaurs originated from facultative or fully bipedal ances-tors. At the moment, this is hampered by the scarce material



Fig. 13. Single hypothesis of basal dinosaur/dinosauromorph relationships in which the recognition of Lagerpeton, Silesaurus, andsauropodomorphs as quadrupedal/facultative bipedal (grey silhouettes) allows an equally parsimonious reconstruction of this gait,relative to a fully bipedal gait (white silhouettes), as ancestral to Dinosauria. White rectangles represent acquisition of bipedalism oneither a quadrupedal/facultative bipedal (C = convergences) or fully bipedal (A = apomorphy) scenario of dinosaur origins. Greyrectangles (R = reversions) represent acquisition of facultative bipedalism/quadrupedalism on a fully bipedal scenario of dinosaurorigins. Silhouettes adapted from various sources.

assigned to most of these forms, specially regarding their forelimb anatomy.

Classical scenarios of dinosaur dietary evolution (Chariget al., 1965) depict independent origins of the major groupsfrom carnivorous ‘‘thecodonts’’. In fact, also in the cladisticparadigm, typical ‘‘carnivorous teeth’’, i.e. pointed, caudallycurved, labiolingually flattened, unexpanded at the base, andwith finely serrated/denticulate keels, are usually acceptedas plesiomorphic for dinosaurs (Gauthier, 1986; Langer &Benton, 2006, but see Barrett, 2000). Indeed, although moreconical in likely piscivorous forms (Sill, 1967; Hungerbuhler,2000), teeth of basal archosaurs (Ewer, 1965; Gower, 2003)and pseudosuchians (Walker, 1964; Gower, 1999; Nesbitt,2003) mainly fit into that pattern. Triassic exceptionsare pseudosuchians that bear modified dentitions towardsomnivory/herbivory/scavenging (Walker, 1961; Parker et al.,2005). Given that known basal dinosauromorph teeth are alllabiolingually flattened, there is a good case that the abovedescribed pattern is indeed ancestral to the group as a whole.Dinosaur dentitions are, however, more heterogeneous.

Until recently, the dentition of non-dinosaur dinosauro-morphs was inferred from the rather fragmentary tooth-bearing bones referred to Lewisuchus admixtus (Romer, 1972a)and Marasuchus lilloensis (Bonaparte, 1975). The isolated par-tial jaw of Lewisuchus may not belong to the taxon, giventhat it was found disarticulated in a concretion with otherarchosaur taxa and seems larger in relation to the holotypeskeleton (PULR 01). The maxilla of Marasuchus (PVL 3870)has been consensually attributed to the taxon (Sereno &Arcucci, 1994), but its teeth are not well preserved enoughfor an accurate inference of its diet. Most of the crowns fitinto the plesiomorphic pattern described above, but morecaudal elements seem slightly distally expanded at the base,making them somewhat ‘‘leaf-shaped’’ (Bonaparte, 1975).Yet, the lack of further dental modifications (see Barrett,2000) precludes the assignment of an alternative diet to

Marasuchus, which probably fed on a variety of small ani-mals. Lately, however, putative basal dinosauromorphs withherbivorous adaptations have been discovered (Dzik, 2003;Ferigolo & Langer, 2007; Irmis et al., 2007a). Silesaurus opolen-

sis and Sacisaurus agudoensis bear an edentulous beak in thelower jaw, plus dental features usually associated with amore herbivorous diet in ‘‘prosauropods’’ and ornithischians(Galton, 1984; Crompton & Attridge, 1986; Sereno, 1991b),but lately given as evidence of omnivory (Barrett, 2000; Irmiset al., 2007b). Accordingly, although a full herbivore ancestryof dinosaurs can be dismissed, there is some evidence that astrictly carnivorous origin was also not the case.

Barrett (2000) comprehensively reviewed early dinosaurfeeding habits, remaining unsure about the ancestraldiet of the group. There is full agreement, however,on the carnivorous habits of theropods and herrerasaurs(Barrett, 2000; Bittencourt, 2008), while basal ornithischians(Irmis et al., 2007b) and typical ‘‘prosauropods’’ (Barrett& Upchurch, 2007) were most probably omnivorous. Thecondition in some small-sized basal saurichians such asEoraptor lunensis and Saturnalia tupiniquim is more uncertain.Eoraptor bore ‘‘leaf-shaped’’ rostral teeth, but its diet was mostprobably still based on small animals. On the other hand,most teeth of Saturnalia are ‘‘leaf shaped’’, and this animalis frequently referred to as bearing herbivorous adaptations(Barrett & Upchurch, 2007). However, the retention of finelyserrated tooth keels (Yates, 2003b) suggests that Saturnalia

was more carnivorous than any basal sauropodomorph.The optimization of dental patterns into current

hypotheses of early dinosaur phylogeny reveals variousalternative scenarios. When forms with less modifieddentition such as Marasuchus and Eoraptor are consideredcarnivores, the reconstruction of the ancestral dinosaurfeeding habit is ambiguous in most cases. Yet, if Eoraptor

plus herrerasaurs are considered basal theropods (Sereno,1999), and Silesaurus opolensis and Sacisaurus agudoensis placedin the sister clade to Dinosauria, then omnivory can be



Fig. 14. Hypotheses of basal dinosaur/dinosauromorph rela-tionships in which either omnivory (A) or carnivory (B) isunambiguously reconstructed as ancestral to Dinosauria. Whiteand black silhouettes/circles, respectively, indicate omnivorousand carnivorous taxa/optimizations. Silhouettes adapted fromvarious sources.

unambiguously regarded as plesiomorphic for the group(Fig. 14A). On the contrary, if Silesaurus and Sacisaurus areconsidered basal ornithischians (Ferigolo & Langer, 2007),and either Eoraptor or herrerasaurs basal saurichians (Langer& Benton, 2006), then dinosaurs are plesiomorphicallycarnivores (Fig. 14B). Yet, it is important to stress that theplesiomorphic dinosaurs tooth morphology does not strictlycompare to that of the typically carnivorous herrerasaursand theropods, or to the omnivorous/herbivorous pattern ofornithischians and ‘‘prosauropods’’. Teeth of basal forms asMarasuchus, Saturnalia, and Eoraptor, are less specialized, andthis was most probably also the case for the dinosaur commonancestor, whether it was a strict carnivore or able to includeplant material in its diet. In this context, each major dinosaurgroup seems to have independently acquired its typicaldental traits, in some cases along with a significant increasein size. Indeed, the omnivorous/herbivorous dentition ofbasal ornithischians and sauropodomorphs are sufficientlydifferent to preclude a common origin (Barrett, 2000). On thecontrary, the carnivorous teeth of herrerasaurs and theropodsare primary homologous (Bittencourt, 2008), and can be usedas evidence for the nesting of the former within the lattergroup. Alternatively, they might have arisen independently,as adaptations to the predatory habits of these animals(Langer & Benton, 2006).

The skin of basal dinosaurs has always been thought asscaly, but this view was recently challenged by the discovery

of Tianyulong confuciusi (Zheng et al., 2009), which providesthe second evidence of an ornithischian with integumen-tary filaments (see also Mayr et al., 2002). Its placementwithin Heterodontosauridae (Zheng et al., 2009), i.e. the sis-ter clade of all other ornithischians except Pisanosaurus (seeSection V.1), shows that at least some very basal ornithis-chians bore integumentary filaments. Accordingly, if theseare actually homologous to the ‘‘protofeathers’’ of theropoddinosaurs, the most recent common ancestor of saurischi-ans and ornithischians would also likely bear this kind ofepidermal coverage. Witmer (2009) is cautious about thedermal or epidermal origin of the integumentary structuresof Tianyulong, and their homology with those of theropods.Yet, further research demonstrating the epidermal origin ofthose filaments, a hypothesis currently supported by theirhollow structure (Zheng et al., 2009), would suggest that earlydinosaurs also bore integumentary filaments. If it was suffi-ciently abundant, the coverage could play a thermoregulatoryfunction (insulation), probably implying a higher thermalinertia (Regal, 1975, 1985; Unwin, 1998; Wu et al., 2004).

V. OUTCOMES OF A RADIATION

(1) Early ornithischian evolution

Although the concept of Ornithischia was defined only sometwenty years later (Seeley, 1888), the first dinosaur classifi-cation schemes already congregated most members of theclade we now know within a ‘‘natural group’’, e.g. OrthopodaCope, 1866. Indeed, ornithischian monophyly remains one ofthe few uncontroversial issues in dinosaur taxonomy (Nopcsa,1923, 1928; Huene, 1956; Romer, 1956, 1966; Steel, 1969;Thulborn, 1971, 1972; Galton, 1972; but see Maryanska,1977), having been fully corroborated by cladistic studies(Sereno, 1999; Butler, 2005; Butler et al., 2007, 2008). Tradi-tional ornithischian diagnostic traits such as the opisthopubicpelvis (Seeley, 1888) and the predentary bone (Marsh, 1894)can not be unambiguously considered apomorphic for thegroup, because their occurrence in the suggested basalmostornithischian, Pisanosaurus mertii, is equivocal (Butler et al.,2008). Yet, various other features have been accepted asdiagnostic for the clade, most of which correspond to modi-fications of the teeth and tooth-bearing bones, related to theacquisition of a more herbivorous diet. These include thepresence of a buccal emargination on the maxilla, into whichcheek musculature may have attached, and several changes inthe shape and arrangement of the teeth, as recently reviewedby Irmis et al. (2007b) and Butler et al. (2008).

Five main ornithischian groups are traditionallyrecognized: Stegosauria, Ankylosauria, Ornithopoda, Pachy-cephalosauria, and Ceratopsia (Thulborn, 1972), but the rela-tionships among these lineages remained ambiguous until thefirst cladistic analyses of the group were performed (Sereno,1984, 1986; Norman, 1984; Maryanska & Osmolska, 1985;Cooper, 1985). Among these, the view advocated by Sereno(1986, 1991b, 1999) was highly influential for nearly two



decades, during which few other phylogenetic studies focus-ing on the basal radiation of Ornithischia were performed.Sereno (1998, 1999) recognizes two main ornithischian splits:Thyreophora and Neornithischia (Table 1). The formerincludes Stegosauria and Ankylosauria, whereas Ceratop-sia and Pachycephalosauria compose Marginocephalia, thesister clade to Ornithopoda within Neornithischia. This gen-eral scheme is accepted by most recent studies (Xu et al.,2006; Butler et al., 2007, 2008), but details of basal formsnesting within each major group remain controversial.

Sereno (1986) combined neornithischians and thyreo-phorans within Genasauria, which encompasses the bulkof the Ornithischia. The first phylogenetic definition ofthe name (Table 1) employed the term Cerapoda in aconnotation equivalent to Neornithischia sensu Sereno (1999).Indeed, Cerapoda was later defined by Weishampel (2004)as ‘‘genasaurs more closely related to Triceratops thanto Ankylosaurus’’. The term was, however, more recentlyabandoned in favour of Neornithischia (Sereno, 1997,1998, 1999, 2005), or used in a radically different node-based concept (Buchholz, 2002; Barrett, Butler & Knoll,2005a; Butler et al., 2008). Originally, non-Genasauriaornithischians included only Lesothosaurus dignosticus, fromthe Early Jurassic Upper Elliot Formation, southern Africa(Thulborn, 1970, 1972; Santa Luca, 1984; Knoll & Battail,2001; Knoll, 2002a, b, c, 2005; Butler, 2005), and Pisanosaurus

mertii (see Section III.1). Despite its plesiomorphic postcranialanatomy, the latter taxon has for a long time been regardedas the sole unequivocal Triassic ornithischian, based on traitsof its partial skull and teeth (Sereno, 1991b; Butler et al., 2007;Irmis et al., 2007b). Historically, Pisanosaurus was related to‘‘fabrosaurids’’ (Thulborn, 1971, 1972), heterodontosaurids(Charig & Crompton, 1974; Bonaparte, 1976; Cooper,1985; Weishampel & Weishampel, 1983; Weishampel, 1984;Crompton & Attridge, 1986), and ‘‘hypsilophodontids’’(Galton, 1972, 1986; Colbert, 1981), but later acceptedas the most basal ornithischian (Novas, 1989; Weishampel& Witmer, 1990; Sereno, 1991b; Butler, 2005; Butler et al.,2007, 2008; but see Norman et al., 2004a). More recently, anon-Genasauria position was also inferred for the only othertwo ornithischian taxa with a Triassic record (Fig. 15A),Heterodontosauridae (Butler et al., 2008) and Eocursor parvus

(Butler et al., 2007). This is partially based on the retentionof several features otherwise atypical for the group suchas a long hand with extensor pits on the metacarpalsand phalanges, longer penultimate phalanges, and stronglyrecurved unguals with prominent flexor tubercles (Butleret al., 2007). Heterodontosaurids are more extensively knownfrom Early Jurassic strata (Fig. 15B), and were traditionallyplaced as the basalmost clade of Ornithopoda (Sereno, 1984,1986; Norman, Witmer & Weishampel, 2004b), althoughunorthodox alternative placements were also proposed inthe cladistic paradigm (Norman, 1984; Cooper, 1985;Maryanska & Osmolska, 1985; Buchholz, 2002; You, Xu& Wang, 2003; Xu et al., 2006). The group is minimallycomposed of Heterodontosaurus tucki, from the Early JurassicUpper Elliot (Santa Luca, 1980) and Clarens (Crompton &

Charig, 1962) formations of South Africa, and Abrictosaurus

consors, from the former unit (Thulborn, 1974). Other post-Triassic heterodontosaurids may include Lycorhinus angustidens

(Gow, 1975), also from the Upper Elliot Formation, unnamedforms from the Kayenta and Clarens formations (Irmis &Knoll, 2008), as well as younger records (Norman & Barrett,2002; Galton, 2005b; Zheng et al., 2009).

From the 1980s onwards, several putative ornithischians(e.g. Galtonia gibbidens, Technosaurus smalli, Revueltosaurus

callenderi, R. hunti, Lucianosaurus wildi, Pekinosaurus olseni,Tecovasaurus murryi, Protecovasaurus lucasi, Crosbysaurus harrisae)were reported from Late Triassic assemblages of NorthAmerica (Hunt, 1989; Hunt & Lucas, 1994; Hunt et al.,1998; Heckert, 2002, 2004) and Europe (Godefroit &Cuny, 1997; Cuny et al., 2000; Galton, 2005b) mostlybased on isolated teeth. Yet, recent studies demonstratedthat the trustworthy Triassic record of the group is muchmore restricted. Parker et al. (2005) reported the first non-dental material of R. callenderi, showing that it represents apseudosuchian rather than an ornithischian. Those authorsalso recognized notable convergences in the dental anatomyof ornithischians and some non-dinosaur archosaurs, e.g.low triangular tooth crowns with expanded base and carinaecomposed of large denticles. Indeed, in a comprehensivereview of the Triassic ornithischian record, Irmis et al. (2007b)reinterpreted most of those isolated teeth as indeterminatearchosauriforms. More recently, Ferigolo & Langer (2007)proposed the ornithischian affinity of the purported basaldinosauromorphs Silesaurus opolensis and Sacisaurus agudoensis

(Fig. 15A). This was partially based on the suggestedhomology of the synapomorphic ornithischian predentarybone to the edentulous tip of the lower jaw seen in bothtaxa, which is formed by independent ossifications in thelatter form. Yet, that proposition was not originally backedup by a numerical cladistic study, and further analysesfailed to recover that hypothesis of relationships (Langer &Benton, 2006; Ezcurra, 2006; Irmis et al., 2007a; Brusatteet al., 2008a).

Until the early eighties, most authors accepted the ‘‘fab-rosaurids’’ as a natural group of basal ornithischians, fre-quently depicted at the stem of either Ornithopoda or a moreinclusive group of non-thyreophoran taxa (Galton, 1978;Norman, 1984). Yet, it became increasingly evident that ‘‘fab-rosaurids’’ congregated a paraphyletic array of early, small-bodied forms (Weishampel & Witmer, 1990; Sereno, 1991b).In his phylogenetic studies, Sereno (1984, 1986, 1991b, seealso Buchholz, 2002; Xu et al., 2006) considered the archety-pal ‘‘fabrosaurid’’ Lesothosaurus dignosticus as the sister taxonto Genasauria. More recently, however, this view was chal-lenged by studies that placed that form within Genasauria(Butler et al., 2007), either as a neornithischian (Butler,2005) or a thyreophoran (Butler et al., 2008). In the lat-ter case, Lesothosaurus would represent the only thyreophoranto lack the typical cortical remodeling of cranial elementsand osteoderms covering the dorsum of the body (Butleret al., 2008). That genus was also reported from the Early



Fig. 15. Phylogenetic relationships and distribution of basal ornithischians. (A) Time-calibrated phylogeny depictingheterodontosaurids as the most basal clade of Ornithischia, based on Butler et al. (2007, 2008); dotted lines indicate uncertain positionof Silesaurus and Sacisaurus according to Ferigolo & Langer (2007). (B) Geographic occurrences of Late Triassic (black squares) andEarly Jurassic (white squares) taxa on a Late Triassic map redrawn from Blakey (2006). (C) More conventional phylogeny depictingheterodontosaurids as ornithopods, based on Sereno (1999); composition of Heterodontosauridae according to Butler et al. (2008).Black silhouettes (roughly at the same scale) adapted from various sources, names applied as in Table 1. In the cladograms, node-and stem -based taxa are respectively indicated by black circles and curved lines.

Jurassic La Quinta Formation, in western Venezuela (Rus-sell et al., 1992), but the specimens can only be referred toindeterminate non-cerapodan ornithischians (Barrett et al.,2008). Other taxa previously regarded as ‘‘fabrosaurids’’(Galton, 1978; Peng, 1997) have been considered of inde-terminate affinity or placed within genasaurian subgroups(Sereno, 1991b; Norman et al., 2004b, c; Butler, 2005).

Early Jurassic ornithischians other than Lesothosaurus dig-

nosticus have a less debated phylogenetic position (Fig. 15).

Scutellosaurus lawleri, from the Kayenta Formation of west-ern USA (Colbert, 1981; Rosenbaum & Padian, 2000),Emausaurus ernsti, from Mecklemberg, Germany (Haubold,1991), and the genus Scelidosaurus, known from the LowerLias of Dorset, England (Owen, 1861, 1863; Martill, Batten& Loydell, 2000; Norman, 2001) and possibly also from theKayenta Formation (Padian, 1989), are consensually con-sidered the basalmost thyreophorans (Sereno, 1999; Butleret al., 2008). Early Jurassic thyreophorans are otherwise only



known from the Lufeng Formation of China (Irmis & Knoll,2008). This includes the type specimens of Bienosaurus lufen-

gensis (Dong, 2001) and Tatisaurus oehleri (Norman, Butler &Maidment, 2007), both of which lack autapomorphic featuresand may represent nomina dubia (Irmis & Knoll, 2008). Exceptfor Lesothosaurus, heterodontosaurids, and thyreophorans,Stormbergia dangershoeki, from the Upper Elliot Formation (But-ler, 2005; but see Knoll et al., 2009), is the sole well knownEarly Jurassic ornithischian. Indeed, although much richerthan during Triassic times, Early Jurassic ornithischian fau-nas are still poorly diverse. Stormbergia dangershoeki seems torepresent the most basal Neornithischia, lacking typical fea-tures otherwise common to the group such as an elongatedprepubic process (Butler, 2005; Butler et al., 2007, 2008).

The divergence time of Ornithischia has been traditionallyconsidered as Late Triassic, given the presence of Pisanosaurus

mertii and various saurischian dinosaurs in South Americandeposits of that age. The phylogenetic hypothesis of Butleret al. (2007) better fits the stratigraphic data, restrictingthe Triassic record of ornithischians to non-Genasauriataxa. Instead, both traditional (Sereno, 1999) and moreunusual (Buchholz, 2002; Xu et al., 2006) arrangementsrequire the existence of long ghost-lineages for Ornithopoda,Marginocephalia, and Thyreophora. The possible positionof two South African forms at the base of both Thyreophoraand Neornithischia (Butler et al., 2008) suggests that the originand early diversification of ornithischians is to be foundin southwestern Gondwana (Rauhut & Lopez-Arbarello,2008), where the Late Triassic record of the group isconcealed (Fig. 15C). On the contrary, the bulk of EarlyJurassic thyreophorans occur in North Pangea, particularlyconsidering the uncertain affinity of the ankylosaur reportedfrom the Kota Formation of India (Nath, Yadagiri & Moitra,2002; Rauhut & Lopez-Arbarello, 2008).

The slightly more conspicuous Middle Jurassic record ofornithischians includes the first stegosaurs and ankylosaurswithin Thyreophora and an array of basal neornithischians.The Lower Shaximiao Formation of China (Peng et al., 2005)has yielded stegosaurs (Maidment & Wei, 2006) and neor-nithischians (Barrett et al., 2005a), while the former groupwas recorded along with a possible pachycephalosaur fromthe Balabansai Formation of Kirghizia (Averianov, Martin& Bakirov, 2005; Averianov, Bakirov & Martin, 2007). InEurope, Middle Jurassic ornithischians are represented bybasal ornithopods (Galton, 1980; Evans & Milner, 1994,Kriwet, Rauhut & Gloy, 1997; Ruiz-Omenaca, Suberbiola& Galton, 2005), stegosaurs (Galton & Powell, 1983, Galton,1990), and ankylosaurs (Galton, 1983a). Indeed, it was notuntil the Late Jurassic and Early Cretaceous that ornithis-chian faunas became abundant, with diverse thyreophorans,ornithopods, ceratopsians, and pachycephalosaurs, especiallyin Laurasian assemblages.

(2) Early sauropodomorph evolution

The sauropodomorph lineage includes dinosaurs with asmall skull, a distally broad humerus, a relatively shorthind limb, plus a series of other skeletal modifications

that support their monophyly in the cladistic paradigm(Sereno, 1999; Yates, 2003b; 2007a; Yates & Kitching, 2003;Langer & Benton, 2006; Upchurch et al., 2007). On thecontrary, as comprehensively reviewed by Sereno (2007a;see also Upchurch, 1997a; Galton & Upchurch, 2004),classical studies (Huene, 1929, 1956; Romer, 1956; Colbert,1964) frequently allocated the basal sauropodomorphsknown as ‘‘prosauropods’’ within a grade of saurischiansfrom which both theropods and sauropods arose. Thename Prosauropoda Huene, 1920, was coined in thatcontext, in reference to a group lying at the base of the‘‘herbivorous-omnivorous’’ branch of Pachypodosauria that,as the name implies, gave rise to sauropods. This includedThecodontosaurus, Plateosaurus, Sellosaurus, and Anchisaurus, butalso Poekilopleuron ‘‘as a blind side stem’’ (Huene, 1920). Onthe contrary, forms at some point regarded as members ofthe sauropodomorph lineage such as Paleosaurus (Benton et al.,2000) and Gresslyosaurus (Yates, 2007a) were allocated to thetheropod-related branch (Huene, 1920). As often mentioned(Benton, 1986b), this was in part due to the mistakenassociation of carnivorous teeth with other ‘‘prosauropod’’skeletal remains (Huene, 1932; Young, 1951). Charig et al.(1965; see also Galton, 1971, 1973) may be said to havesettled the current concept of ‘‘Prosauropoda’’, congregatingvarious early saurischians of the sauropodomorph lineage. Asproperly put by Sereno (2007a), ‘‘prosauropods’’ representedthe first grand radiation of dinosaurs sharing minimalmorphological coherence. Basal sauropodomorphs radiatedrelatively fast during Late Triassic times, becoming thedominant terrestrial herbivores/omnivores from Norian toEarly Jurassic landscapes (Upchurch, 1997a).

Sereno (2007a) selected from the universe of basalsauropodomorphs a subset of five taxa termed ‘‘core-prosauropods’’, which should carry the name Prosauropoda,if found to represent a monophylum exclusive of sauropods.A comparable application is seen in Yates & Kitching (2003)and Upchurch et al. (2007). However, the first phylogeneticdefinition of the name was proposed under the orthodoxy ofa monophyletic ‘‘Prosauropoda’’ as to include ‘‘Thecodon-tosauridae, Plateosauridae (Anchisauridae), Melanosauridae[sic], and all Sauropodomorpha closer to them than toSauropoda’’ (Upchurch, 1997a; Sereno, 2005). Based on thetype-genera of those family rank names, this definition is notapplicable to the current framework of sauropodomorph evo-lution, since a clade that includes Thecodontosaurus, Plateosaurus,Anchisaurus, and Melanorosaurus also includes sauropods. Con-sidering the criteria adopted here (Section II.2), that defi-nition has precedence over following ones that arbitrarilyselect either a single (Sereno, 1998; Galton & Upchurch,2004) or various ‘‘prosauropods’’ (Sereno, 2007a) as inter-nal specifiers. Indeed, the definition of Upchurch (1997a)is to be kept and applied under an eventually revitalizedframework of ‘‘prosauropod monophyly’’, while newly pro-posed names should be used to designate major subgroupsof Sauropodomorpha, as seen in Yates (2007a) and Smith &Pol (2007).



Early evolutionary studies of basal sauropodomorphs(Charig et al., 1965; Galton, 1971, 1976) broadly dis-criminate three main ‘‘prosauropod’’ groups: one includ-ing more gracile forms, the so-called ‘‘narrow-footedprosauropods’’, termed either Thecodontosauridae orAnchisauridae; a group of more ‘‘typical’’, Plateosaurus-relatedforms; and a group of bulky quadrupeds, frequently termedMelanorosauridae. The sauropod affinity of the latter groupwas often advocated (Colbert, 1964; Cooper, 1981; Bona-parte, 1986), hinting at ‘‘prosauropod’’ paraphyly. Indeed,the first cladistic approach to sauropodomorph evolutionreproduced that scheme (Gauthier, 1986), allocating The-

codontosaurus antiquus and Efraasia minor at the base of theclade, and the ‘‘broad-footed’’ forms, especially Riojasaurus

incertus, closer to Sauropoda. The succeeding cladistic studies(Sereno, 1989, 1999; Galton, 1989; Benton et al., 2000;Galton & Upchurch, 2004), however, failed to recovera similar paraphyletic array of ‘‘prosauropods’’. Instead,they pointed towards a different picture, in which most, ifnot all ‘‘prosauropods’’ represented the monophyletic sis-ter taxon to Sauropoda. This scheme is reminiscent ofpre-cladistic approaches (Charig et al., 1965; Cruickshank,1975; Van Heerden, 1978) that partially relied on the sup-posed irreversibility of some features of the ‘‘prosauropod’’foot, and on the uniqueness of their hand, in order todiscard their bearing on the origin of sauropods (but seeYates, 2003b; Sereno, 2007a). More recently, ‘‘prosauro-pod’’ monophyly was deemed an analytical artifact derivedfrom poor taxon sampling, that overlooked basal and near-sauropod sauropodomorphs (Yates, 2003b; see also Sereno,2007a). Indeed, most recent studies (Yates & Kitching, 2003;Pol, 2004; Smith & Pol, 2007; Yates, 2007a, b), includ-ing those performed by previous proposers of ‘‘prosauropodmonophyly’’ (Upchurch et al., 2007; Sereno, 2007a) tendto agree that at least some forms previously assigned to‘‘Prosauropoda’’ are basal to the bulk of sauropodomorphs,and that others are closely related to the sauropod radiation(Fig. 16A). Evidently, these hypotheses are not fully congru-ent with one another, but important common points areseen, as outlined below.

The most recently proposed basal sauropodomorphphylogenies (Pol, 2004; Yates, 2007a, b; Upchurch et al.,2007) agree that the Late Triassic Saturnalia tupiniquim,Pantydraco caducus, Thecodontosaurus antiquus, and Efraasia minor

are amongst the most basal members of the lineage, whereasPanphagia protos may be the basal-most member (Martinez& Alcober, 2009). Upchurch et al. (2007) also includedMussaurus patagonicus in that basal grade, but the taxonwas given a more derived position by Pol (2004), Pol &Powell (2005, 2007b), and Sereno (2007a) based on first-handexamination of a more complete set of specimens. Such abasal position was also inferred, in an admittedly tentativefashion, to the newly described Pradhania gracilis from theEarly Jurassic Upper Dharmaram Formation, India (Kuttyet al., 2007). The relative positions of the other forms arenearly consensual, with Saturnalia basal to Thecodontosaurus

and Pantydraco (but see Galton & Upchurch, 2004), and

Efraasia closer to other sauropodomorphs (Fig. 16A). Thesetaxa represent the first radiation of the sauropodomorphlineage, retaining various morphological traits of theirbasal saurischian/basal dinosauriform precursors (Barrett& Upchurch, 2007) such as the smaller size (adults are lessthan 4 m in length) and, at least facultative, bipedality. Othersimplesiomorphies include a relatively long hand, a partiallyclosed acetabulum, a distal femur lacking a well-developedextensor depression, metatarsals I and II closely appressed,plus several other skeletal features (Yates, 2003b; Yates &Kitching, 2003; Smith & Pol, 2007; Upchurch et al., 2007).In addition, some of these forms possess novel herbivorousadaptations such as a higher number of coarsely denticulatedteeth, although they might have retained an omnivorousdiet (Barrett, 2000). The geographic distribution of thesebasal sauropodomorphs, along with that of slightly morederived forms (Pol, 2004; Yates, 2007a), suggests an initialradiation of the clade restricted to western Pangea (Fig. 16C).Considering the node-based definition of Sauropodomorphaand the (by typification) inclusion of Thecodontosaurus as aninternal specifier of Prosauropoda, Langer (2002) proposedthat Saturnalia should be excluded from Sauropodomorpha,and considered instead as a taxon on its stem lineage.

The relationships of sauropodomorphs more derived thanEfraasia minor are far from consensual. In fact, severalpossible arrangements recently have been proposed (seevarious articles in Barrett & Batten, 2007; especially Sereno,2007a). In most of them, however, a relatively stable setof taxa is placed closely related to traditional sauropods(Fig. 16A), minimally including Norian-Rhaetian forms suchas Camelotia borealis, Melanorosaurus readi, Blikanasaurus cromptoni,and Lessemsaurus sauropoides, as well as Antetonitrus ingenipes,which was already first described as a sauropod. This roughlycorresponds to the classical radiation of ‘‘melanorosaurids’’,composed of usually larger (6.5-10 m), more herbivorous‘‘prosauropods’’ (Barrett, 2000). Most of these may haveadopted a fully quadrupedal gait (Yates & Kitching, 2003),although facultative bipedality is still suggested for severalforms (Barrett & Upchurch, 2007). These taxa share arange of traits with eusauropods, including short and highdorsal centra, an increased number of sacral vertebrae, alonger manual digit I with a straighter ungual, broader non-terminal manual phalanges, and a straighter femur, ellipticalin cross section and bearing distally displaced lesser andfourth trochanters (Yates & Kitching, 2003; Yates, 2007a,Upchurch et al., 2007; Pol & Powell, 2007a). Some authors(Yates & Kitching, 2003; Smith & Pol, 2007; Yates, 2007a, b)include all or some of these forms within Sauropoda, given thestem-based definition of the taxon as to include forms closerto Saltasaurus loricatus than to the archetypal ‘‘prosauropod’’Plateosaurus engelhardti (Wilson & Sereno, 1998, Sereno, 1999;Upchurch et al., 2007), or alternative arbitrary attemptsto mimic more traditional/current placement of formseither within or outside the group (Yates, 2007a; Sereno,2007a). However, Sauropoda was first phylogeneticallydefined by Salgado et al. (1997) in a node-based fashion(Table 1) that may exclude forms regularly assigned to the



Fig. 16. Phylogenetic relationships and distribution of basal members of the sauropodomorph lineage. (A) Time-calibratedphylogeny depicting ‘‘core prosauropods’’ as a paraphyletic group, based on Yates (2007a, b), Smith & Pol (2007), and Martinez &Alcober (2009); asterisk indicates alternative placement of Yunannosaurus according to Pol (2004). (B) Alternative phylogeny depicting‘‘core prosauropod’’ monophyly, based on part of the topology proposed by Upchurch et al. (2007). (C) Geographic occurrences ofLate Triassic (black squares) and Early Jurassic (white squares) taxa on a Late Triassic map redrawn from Blakey (2006). Namesapplied as in Table 1; black silhouettes (roughly at the same scale) adapted from various sources. In the cladograms, node- and stem-based taxa are respectively indicated by black circles and curved lines.

clade such as Isanosaurus attavipachi, Gongxianosaurus shibeiensis,Chinshakiangosaurus chuhghoensis, and Kotasaurus yamanpalliensis

(see table 2 in Upchurch et al., 2007). The latter three taxabelong into the Early Jurassic (He, Li & Cai, 1988; Yadagiri,2001; Upchurch et al., 2007) radiation of sauropodomorphsthat also includes some European (Wild, 1978) and NorthAfrican (Allain et al., 2004) forms. These are derived froman already widespread set of related Triassic taxa, leading tothe well-established diversity of Mid-Late Jurassic sauropods(Rauhut & Lopez-Arbarello, 2008).

Given such a more restrictive definition of Sauropoda,Yates (2007a) proposed the name Massopoda (= Sauropodasensu Wilson & Sereno, 1998) also to encompass the

‘‘prosauropod’’ stem leading to that group, exclusive of Pla-

teosaurus engelhardti. In parallel, Sereno (2005) named a cladecomposed of sauropods plus some related ‘‘prosauropods’’as Sauropodiformes (Table 1). Apart from the taxa discussedabove, both Massopoda and Sauropodiformes probably alsoinclude Mussaurus patagonicus (Bonaparte & Vince, 1979; Pol& Powell, 2007b) and Jingshanosaurus xinwaensis, from theEarly Jurassic Lufeng Formation of Yunnan, China (Zhang& Yang, 1994), a form accepted by most authors as sharingsauropod affinities (Pol, 2004; Upchurch et al., 2007; Yates,2007a). Additionally, newly discovered Early Jurassic formsas Lamplughsaura dharmaramensis from the Upper Dharmaram



Formation (Kutty et al., 2007), and an undescribed Argen-tinean taxon (Pol & Garrido, 2007) may also belong to thesauropod stem. Other Early Jurassic forms such as Yunan-

nosaurus huangi, also from the Lufeng Formation (Young,1942), and the ‘‘prosauropod’’ from the Portland Forma-tion of eastern North America (Anchisaurus polyzelus sensu

Yates, 2004; Ammosaurus major sensu Sereno, 2007a), havehighly controversial affinities, and two antipodal scenarioshave been presented (but see Pol, 2004). They may lineup with other sauropod-related forms (Yates, 2007a, b) orbelong to a clade of typical ‘‘prosauropods’’ (Upchurch et al.,2007). There is limited evidence in favour of a monophyleticgroup of ‘‘core prosauropods’’ (Sereno, 2007a), minimallyencompassing Plateosaurus engelhardti, Riojasaurus incertus, Mas-

sospondylus carinatus, Lufengosaurus huenei, and Coloradisaurus brevis

(Fig. 16B). Potential apomorphic traits of this clade includemodifications in the neurovascular foramina of the maxilla,the manus (carpal II does not completely cover the proxi-mal end of metacarpal II, metacarpal V bears an expandedproximal end divided into two articular surfaces, first pha-lanx of manual digit I twisted by at least 60◦), and foot(metatarsal IV with expanded proximal end). On the con-trary, Pol (2004) and Yates (2007a, b) split that group intoa successive array of three to five lineages on the stem toSauropodiformes sensu Sereno (2007a), where forms relatedto Plateosaurus and Riojasaurus are considered more basal(Fig. 16A). Depending on the position of these forms relativeto other sauropodomorphs is the application of names suchas Plateosauria and Anchisauria, e.g. compare Yates (2007a)and Upchurch et al. (2007).

Either as a clade or grade, ‘‘core prosauropods’’ mayrepresent a biological unit, playing similar roles in the LateTriassic-Early Jurassic ecosystems in which they occurred.Barrett & Upchurch (2007) reviewed the palaeobiology ofthese forms, highlighting some of their ecological adaptations.These include larger size (2.5-10 m) compared to more basalsauropodomorphs, and greater reliance on an herbivorousdiet, although facultative omnivory was not discarded. ‘‘Coreprosauropods’’ were also capable of bipedal locomotion(Cooper, 1981; Christian & Preuschoft, 1996; Bonnan& Senter, 2007), although larger forms were probablyobligatory quadrupeds. This is the case for Riojasaurus incertus,previously connected to the origin of sauropods (Bonaparte,1972; Gauthier, 1986). As a whole, that clade/gradecongregates a relatively high diversity of Late Triassic forms,including Ruehleia bedheimensis and the species of Plateosaurus

in Europe/Greenland, Riojasaurus incertus and Unaysaurus

tolentinoi in South America, as well as Eucnemesaurus fortis

and Plateosauravus cullingworthi in South Africa. Other ‘‘core-prosauropods’’ may fit into Massospondylidae, a clade thatincludes the Triassic Coloradisaurus brevis (Yates & Kitching,2003), the Early Jurassic Glacialisaurus hammeri (Smith & Pol,2007) from the Hanson Formation, Antarctica, and a possibleset of Chinese forms (see Pol, 2004), minimally includingLufengosaurus huenei from the Lufeng Formation (Barrett,Upchurch & Xiao-Lin, 2005b). Its type genus Massospondylus

(M . carinatus) is known from the Upper Elliot Formation

(Cooper, 1981; Gow, Kitching & Raath, 1990; Sues et al.,2004, Reisz et al., 2005) and other stratigraphic units insouthern Africa (Cooper, 1981; Galton & Upchurch, 2004),but not in North America (Attridge, Crompton & Jenkins,1985; Sues et al., 2004). More recently, its sister taxon,Adeopapposaurus mognai, was described from the Lower JurassicCanon del Colorado Formation, Argentina (Martinez, 2009).

(3) Early theropod evolution

The name Theropoda was coined by Marsh (1881) asa new suborder of carnivorous dinosaurs, but its statusas a ‘‘natural group’’ was rejected for the first half ofthe last century. At the time, gracile members of thegroup, the so-called ‘‘coelurosaurs’’, were considered, alongwith ‘‘prosauropods’’, the ‘‘basal stock’’ from which bothsauropods and derived theropods evolved (Huene, 1914,1920, 1932; Romer, 1956). Theropod monophyly was hintedat by Matthew & Brown (1922), firmly established by Col-bert (1964), in an arrangement widely accepted since (Chariget al., 1965; Romer, 1966; Colbert & Russell, 1969; Ostrom,1978; Steel, 1970; Bakker & Galton, 1974), and corrobo-rated by pioneering phylogenetic studies (Thulborn, 1984;Gauthier, 1986; Novas, 1992b; Holtz, 1994; Sereno, 1999).Indeed, taxa consensually assigned to the group share a seriesof typical traits, e.g. promaxillary foramen; well-developedpneumatization in cervical and cranial trunk vertebrae;manus with reduced metacarpal I, slender metacarpal III,and reduced/absent digits IV and V; ilium with promi-nent supracetabular crest and preacetabular ala; tibia withmarked cnemial and fibular crests; transversely compressedcalcaneum; foot with reduced outer digits (Rauhut, 2003;Ezcurra & Cuny, 2007; Ezcurra & Novas, 2007a).

As discussed in Section III.2, the most contentiousaspect of early theropod evolution is the possible nestingof various Triassic forms within the group (Fig. 9). Thisis particularly the case fot Eoraptor lunensis (Sereno et al.,1993) and herrerasaurs (Sereno & Novas, 1992), but alsofor other taxa such as Guaibasaurus candelariensis (Langer et al.,2007a), Agnosphitys cromhallensis, and Chindesaurus bryansmalli

(Yates, 2007a). Indeed, these forms apart, the Noriancoelophysoid ‘‘Camposaurus arizonensis’’ represents the oldesttheropod (Hunt et al., 1998; Nesbitt et al., 2007), which wouldmake Theropoda the only major dinosaur lineage lacking awell-defined Ischigualastian record. Moreover, based on thecurrent knowledge of theropod diversity, and not consideringthe above-mentioned taxa as members of the group, thestem-based Theropoda would be as inclusive as the node-based Neotheropoda (Table 1). This name was first usedby Bakker (1986) to combine theropods more derived than‘‘podokesaurids’’, but phylogenetically defined by Sereno(1998) as a more inclusive group. Yet, Neotheropoda remainsa useful name under alternative arrangements (Fig. 9) and/orif new forms are found to belong to its stem (see Nesbitt &Chatterjee, 2008; Martinez et al., 2008). Although a lessinclusive Neotheropoda (Padian et al., 1999; Wilson et al.,2003) seems more useful in the current orthodoxy, and moreproperly translates the original meaning of the name (Bakker,



1986), the definition proposed by Sereno (1998) has historical‘‘priority’’.

Another controversial aspect of early theropod evolution isthe possible monophyly of the oldest neotheropods, groupedwithin Ceratosauria and/or Coelophysoidea. Indeed, the firstcladistic analyses of theropod relationships identified twomain neotheropod lineages, Ceratosauria and Tetanurae(Gauthier, 1986; Rowe, 1989; Rowe & Gauthier, 1990).In turn, Ceratosauria was divided into two branches, theLate Triassic-Early Jurassic Coelophysoidea (i.e. Coelophysis,Dilophosaurus, and their kin) and the Jurassic-CretaceousNeoceratosauria, including Ceratosaurus and abelisauroids(Novas, 1992a; Sereno, 1997, 1999; Holtz, 2000; Coria &Salgado, 2000). This arrangement was accepted during mostof the 1990s, but the vast majority of more recent studiesconsider neoceratosaurs more closely related to tetanuransthan to coelophysoids, challenging the monophyly ofthe traditional Ceratosauria (Carrano, Sampson & Forster,2002; Carrano, Hutchinson & Sampson, 2005; Rauhut,2003; Sereno, Wilson & Conrad, 2004; Yates, 2005; Ezcurra,2006; Ezcurra & Novas, 2007a; Ezcurra & Cuny, 2007; Smithet al., 2007; Carrano & Sampson, 2008; but see Tykoski &Rowe, 2004; Tykoski, 2005; Allain et al., 2007). In fact,Ceratosauria was node-based defined by Rowe & Gauthier(1990) as ‘‘including Ceratosaurus nasicornis, Dilophosaurus

wetherilli, Liliensternus liliensterni, Coelophysis bauri, Syntarsus

rhodesiensis, Syntarsus kayentakatae, Segisaurus halli, Sarcosaurus

woodi, and all other taxa stemming from their most recentcommon ancestor’’, based on a phylogenetic frameworkin which these forms compose a monophylum exclusiveof tetanurans. On the contrary, in the current orthodoxy(Fig. 17), Ceratosauria would point to the same node asNeotheropoda, circumscribing a much more inclusive group,and is not employed here. Instead, the term AverostraPaul, 2002, as phylogenetically defined by Ezcurra & Cuny(2007), designates the clade composed of Tetanurae plusNeoceratosauria (Table 1).

Early phylogenetic studies grouped all Triassic andEarly Jurassic neotheropods within the Coelophysoideaclade. This included Dilophosaurus wetherilli, as the sistertaxon to Liliensternus liliensterni plus Coelophysidae (Rowe& Gauthier, 1990; Holtz, 1994). The latter group isminimally composed of Late Triassic (Colbert, 1989; Rauhut& Hungerbuhler, 2000) and Early Jurassic (Raath, 1969;Rowe, 1989) Coelophysis-‘‘Syntarsus’’-related forms (Paul, 1988;Bristowe & Raath, 2004), but may also include Segisaurus

halli and Procompsognathus triassicus (Sereno, 1997, 1999;Tykoski & Rowe, 2004; Knoll, 2008). With the additionof newly described forms, such as Zupaysaurus rougieri andLophostropheus airelensis, various cladisitic analyses (Tykoski &Rowe, 2004; Carrano et al., 2005; Tykoski, 2005; Ezcurra,2006; Ezcurra & Novas, 2007a; Ezcurra & Cuny, 2007)also recovered a monophyletic Coelophysoidea sensu lato

(Fig. 17B). As such, the group can be diagnosed by severalcraniomandibular features such as a flexible articulationbetween premaxilla and maxilla marked by a prominentsubnarial diastema, an expanded rostral end of the dentary,

reduced serrations on premaxillary teeth, and enlarged (fang-like) teeth in the rostral portion of the dentary, as well asby a peculiar pattern of femoral dimorphism (Tykoski &Rowe, 2004; Ezcurra & Novas, 2007a, Ezcurra & Cuny,2007). However, the monophyletic status of that groupwas questioned by Rauhut (2003), who found Dilophosaurus

more closely related to tetanurans and neoceratosaurs thanto coelophysids. Along with other forms, those taxa sharecranial details such as a lacrimal fenestra, a dorsoventrallyelongated orbit, and a reduced tooth count occupyinga shorted portion of the maxilla, modifications in theretroarticular process of the lower jaw, and a higher astragalarascending process (Smith et al., 2007). A similar hypothesis ofrelationship was advocated by Yates (2005), which consideredDracovenator regenti as closely related to Dilophosaurus wetherilli.Smith et al. (2007) also found other Early Jurassic taxasuch as Cryolophosaurus ellioti and ‘‘Dilophosaurus’’ sinensis

as members of such a ‘‘Dilophosaurus clade’’ (Fig. 17A),partially characterized by the presence and/or differentialconfiguration of the dorsal crests of the skull. Yates (2005)also assigned Zupaysaurus to that clade, but the phylogeneticposition of that taxon is highly controversial within non-averostran neotheropods (Carrano et al., 2005; Smith et al.,2007; Ezcurra & Novas, 2007a). Clearly, as stem-based-defined by Sereno (1998), the inclusivity of Coelophysoideais dependent on the adopted evolutionary framework. Itmay include only small to medium-sized forms similarto Coelophysis-‘‘Syntarsus’’ (Fig. 17A), or also congregate anEarly Jurassic radiation of large-bodied Dilophosaurus-liketaxa (Fig. 17B).

Neotheropods experienced a rapid diversification duringthe Norian-Rhaetian and Early Jurassic, achieving a broaddistribution over west Pangea (Fig. 17C). This early radiationwas mostly represented by sensu lato ‘‘coelophysoids’’, i.e.non-averostran neotheropods. Their Norian representativesinclude small to medium-sized forms reported from westernUSA (Colbert, 1989; Carpenter, 1997; Nesbitt et al., 2007)and Europe (Sereno & Wild, 1992; Rauhut & Hungerbuhler,2000; Allen, 2004), most of which are also sensu sticto

Coelophysoidea (Fig. 17B), and perhaps also larger formssuch as Zupaysaurus rougieri (Arcucci & Coria, 2003).Close to the Triassic-Jurassic boundary, theropod remainsbecome scarce and basically only Lophostropheus airelensis

is known (Ezcurra & Cuny, 2007; but see Dzik et al.,2008). Tetanurans previously have been reported from LateTriassic outcrops, but these records are not conclusive. Thesupposed bird Protoavis texensis (Chatterjee, 1991) has beenrecently reinterpreted as a chimaera (Nesbitt et al., 2007),with some elements of ‘‘coelophysoid’’ affinities, but notof tetanuran nature. Otherwise, ‘‘Zanclodon’’ cambrensis wasassigned to Tetanurae (Holtz, Molnar & Currie, 2004),but the available material does not differ from those ofnon-averostran theropods.

During the Early Jurassic (Fig. 17C), sensu sticto coelo-physoids continue to be well represented in western USA,including Segisaurus halli from the Navajo Sandstone (Camp,1936; Carrano et al., 2005) and ‘‘Syntarsus’’ kayentakatae from



Fig. 17. Phylogenetic relationships and distribution of basal theropods. (A) Time calibrated phylogeny depicting ‘‘coelophysoids’’as a paraphyletic group, based on Smith et al. (2007); position of Lophostropheus according to Ezcurra & Cuny (2007); position ofProcompsognathus and relations within Coelophysidae according to Tykoski & Rowe (2004); asterisk indicates alternative placementof Zupaysaurus as sister taxon to Dracovenator, according to Yates (2005); dotted lines indicate uncertain position of herrerasauridsand Eoraptor according to Sereno (1999) and Guaibasaurus according to Langer & Benton (2006). (B) Alternative phylogeny depicting‘‘coelophysoid’’ monophyly, based on Ezcurra & Cuny (2007) and Ezcurra & Novas (2007a); asterisk indicates alternative placementof Zupaysaurus according to Carrano et al. (2005). (C) Geographic occurrences of Late Triassic (black squares) and Early Jurassic(white squares) taxa on a Late Triassic map redrawn from Blakey (2006). Names applied as in Table 1; black silhouettes (roughly atthe same scale) adapted from various sources. In the cladograms, node- and stem -based taxa are respectively indicated by blackcircles and curved lines.

the Kayenta Formation (Rowe, 1989), both in Arizona. Inaddition, ‘‘Coelophysis’’ rhodesiensis is known from the UpperElliot Formation of South Africa (Raath, 1980) and especiallyfrom the Forest Sandstone of Zimbabwe (Raath, 1969). Fur-ther Jurassic records of ‘‘coelophysoids’’ are known fromChina (Irmis, 2004), Mexico (Munter & Clark, 2006), andpossibly Europe (Andrews, 1921; Carrano & Sampson, 2004).

Larger forms attributed or not to the Dilophosaurus-cladealso retain a broad distribution. These include Dilophosaurus

wetherilli from the Kayenta Formation (Welles, 1984), Dra-

covenator regenti from the Upper Elliot Formation (Yates,2005), Cryolophosaurus ellioti from the Hanson Formationof the Transantarctic Mountains (Hammer & Hickerson,1994; Smith et al., 2007), and ‘‘Dilophosaurus’’ sinensis from



the Lufeng Formation of China (Hu, 1993). In addition,the oldest averostrans are known from Early Jurassic assem-blages. This includes Berberosaurus liassicus, a neoceratosaurfrom Morocco (Allain et al. 2007; but see Xu et al., 2009),and the very dubious record of a therizinosauroid jaw in theLufeng Formation (Zhao & Xu, 1998; Xu, Zhao & Clark,2001; Rauhut, 2003). In any case, as sister taxon to Neo-ceratosauria, a tetanuran ghost lineage might be inferred forthe latest Early Jurassic (Fig. 17A). Further Early Jurassictheropods were recorded from the La Quinta Formation ofVenezuela (Moody, 1997) based on teeth that can not beallocated in a less inclusive clade.

Middle Jurassic dinosaur-bearing assemblages are rare(Rauhut & Lopez-Arbarello, 2008), but the available datashow that the composition of theropod faunas changeddrastically relative to those of Late Triassic and EarlyJurassic age. ‘‘Coelophysoids’’ disappear completely from thefossil record, and tetanurans became the dominant forms.Apart from indeterminate theropod remains from NorthAfrica (Monbaron, Russell & Taquet, 1999) and Madagascar(Flynn et al., 2006), Middle Jurassic forms include a probablebasal neoceratosaur from Australia (Long & Molnar, 1998;Rauhut, 2005a), and basal tetanurans form Argentina(Rauhut, 2005a), Europe, and China (Holtz et al., 2004;Smith et al., 2007). On the other hand, neoceratosaurs arebetter known from Late Jurassic and especially Cretaceousdeposits (Carrano & Sampson, 2008; Xu et al., 2009).

VI. CONCLUSIONS

(1) The oldest unequivocal records of Dinosauria are ofLate Triassic age (approximately 230 Mya). These wereunearthed from rocks accumulated over extensional riftbasins in Argentina, Brazil, Zimbabwe, and India. Thebetter known early dinosaurs are Herrerasaurus ischigualastensis,Pisanosaurus mertii, Eoraptor lunensis, and Panphagia protos fromthe Ischigualasto Formation, northwestern Argentina, andStaurikosaurus pricei and Saturnalia tupiniquim from the SantaMaria Formation, south Brazil. Other dinosaur records ofequivalent age are either more fragmentary or of dubiousaffinities, hinting at a south Pangea origin of the group. Nouncontroversial dinosaur body fossils are known from olderstrata, but a possible Middle Triassic origin of the lineagemay be inferred from both the footprint record and its sistergroup relation to Ladinian basal dinosauromorphs.

(2) Dinosauria is by definition a monophyletic group that,in the present orthodoxy, combines saurischians and ornithis-chians to the exclusion of other major archosaur groups suchas pterosaurs, phytosaurs, and crocodylomorphs. The firstphylogenetic definition to fit the current understanding ofDinosauria as a node-based taxon solely composed of mutu-ally exclusive Saurischia and Ornithischia was given as ‘‘alldescendants of the most recent common ancestor of birdsand Triceratops’’. This definition should be followed untilmore specific provisions are given by the PhyloCode.

(3) Dinosaurs are nested within the bird-line of archosaursalong with pterosaurs (possibly), Scleromochlus taylori, and basalDinosauromorpha, the phylogeny of which is in state offlux. It includes the archetypal Marasuchus lilloensis, a diver-sity of more basal forms such as Lagerpeton and Dromomeron,as well as silesaurids: a possibly monophyletic group thatcombines Mid-Late Triassic basal dinosauromorphs thatmay represent sister taxa to Dinosauria. Recent cladisticanalyses of basal dinosaur relationships agree in variouskey points: (1) Pisanosaurus mertii is a basal ornithischian;(2) Herrerasaurus ischigualastensis and Staurikosaurus pricei belongin a monophyletic Herrerasauridae; (3) Guaibasaurus candelar-

iensis, Eoraptor lunensis, and herrerasaurids are saurischians;(4) Saurischia includes two main groups, Sauropodomor-pha and Theropoda; and (5) Saturnalia tupiniquim is a basalmember of the sauropodomorph lineage, a position alsoinferred for the recently described Panphagia protos. On thecontrary, several aspects of basal dinosaur phylogeny remaincontroversial, including the position of silesaurids as basalornithischians or non-dinosaur dinosauromorphs; the posi-tion of herrerasaurids, Eoraptor, and Guaibasaurus as basaltheropods or basal saurischians; and the affinity and/or valid-ity of various fragmentary taxa such as Agnosphitys cromhallensis,Alwalkeria maleriensis, Chindesaurus bryansmalli, Saltopus elginensis,Spondylosoma absconditum, and Teyuwasu barberenai.

(4) The identification of dinosaur apomorphies ishampered by the incompleteness of the skeletal remainsattributed to most basal dinosauromorphs, the skull and forelimb of which are particularly poorly known. Nonetheless,to the exclusion of silesaurids, Dinosauria can be diagnosedby a suite of derived traits, namely: foramen-sized post-temporal fenestra; epipophyses on cranial cervical vertebra;long deltopectoral crest; open acetabulum; arched dorsaliliac margin; femoral head inturned and distinctly offset fromthe shaft; asymmetrical fourth trochanter; astragalus withacute anteromedial corner, broad ascending process, andreduced fibular articulation; and proximally flat lateral distaltarsal. On the contrary, long-standing dinosaur apomorphiessuch as the absence of a postfrontal, the presence of morethan two sacral vertebrae, reduced manual digits IV andV, modified ‘‘lesser trochanter’’, and metatarsals II and IVsubequal in length do not unambiguously diagnose the group.The prevalence of dinosaur diagnostic traits related to thepelvic girdle and limb may reflect the better preservationof these structures in their sister taxa, but may also suggestthat these anatomical parts suffered most of the changes seenin the early dinosaur skeleton. Some of these traits can berelated to the acquisition of an erect bipedal gait, which hastraditionally been suggested to represent a key adaptationthat allowed, or even promoted, dinosaur radiation in theLate Triassic.

(5) Contrary to the classical ‘‘competitive’’ model,dinosaurs did not gradually replace other terrestrial tetrapodsover the Late Triassic. Yet, opportunistic and competitivescenarios are not mutually exclusive, and species interactionmay have played a partial role in the rise of dinosaurs,which can be said to have consisted of three landmark



moments, separated by controversial (Carnian-Norian,Triassic-Jurassic) extinction events. The Carnian earlydiversification did not occur in an empty ecospace but despitethe abundance and diversity of contemporary tetrapods.The Norian increase in dominance might be connected toclimatic/floristic changes and to the extinction of herbivorousforms such as rhynchosaurs, but the timing of these eventsneeds further investigation. The subtle Jurassic diversificationseems to have occurred in the aftermath of an extinctionevent, and might be an example of opportunistic radiationinto released ecospace, with the origin of neornithischiansand thyreophorans, the heterodontosaurid diversity peak,the rise of the Dilophosaurus-clade, and further acquisition oftypical graviportal traits among sauropods.

(6) It is traditionally believed that dinosaurs arose frombipedal and carnivorous forms, but evidence gatheredfrom newly discovered basal dinosauromorphs indicatethat quadrupedalism and omnivory/herbivory can not bediscarded as possible ancestral traits of the group. At themoment, however, most evidence points towards a fullybipedal origin of dinosaurs. On the contrary, dependingon the accepted hypothesis of relationships, the ancestraldinosaur diet can be reconstructed as either carnivorousor omnivorous. In any case, the plesiomorphic toothmorphology of dinosaurs does not strictly compare to thetypical carnivorous or omnivorous/herbivorous patterns ofmore derived members of the group. Indeed, each majordinosaur group seems to have independently acquired itstypical set of dental traits.

(7) The phylogenetic relationships of the basal membersof each major dinosaur group have recently been reeval-uated in the light of new evidence. Among ornithischians,unorthodox placements inside and outside Genasauria wereproposed for Lesothosaurus diagnosticus and Heterodontosauri-dae, respectively. Within saurischians, both ‘‘Prosauropoda’’and ‘‘Ceratosauria’’ were regarded as paraphyletic in theirbroader understanding. Yet, ‘‘core prosauropods’’ and coelo-physoids may still represent smaller clades at the base ofSauropodomorpha and Theropoda, respectively.

(8) Whereas the oldest dinosaurs were geographicallyrestricted to south Pangea, including rare ornithischiansand more abundant basal members of the saurischianlineage, the group achieved a nearly global distributionby Norian/Rhaetian times, especially with the radiationof saurischian groups such as ‘‘prosauropods’’ and coelo-physoids. This suggests an ‘‘out of south Pangea’’ model ofdinosaur radiation, but no model is better than the evidenceupon which it stands. In this case, the evidence is restrictedto a handful of fossils from scattered areas around the world,and more prospection work is needed in order to build a morereliable scenario of dinosaur origins and basal radiation.

VII. ACKNOWLEDGEMENTS

We thank the following for permission to examine specimensmentioned in this paper and help during this work: Angela

Milner and Sandra Chapman (BMNH); Rainer Schoch andRupert Wild (SMNS); Ricardo Martinez and Oscar Alcober(UNSJ); Dave Unwin (MB); Pat Holroyd, Randy Irmis,and Kevin Padian (UCMP); Michael Maisch (GPIT); JaimePowell and Judith Babot (PVL); Mike Raath (QVM); JerzyDzik and Tomasz Sulej (ZPAL); Maria Claudia Malabarba(MCP); Jorge Ferigolo and Ana Maria Ribeiro (FZB/RS);and Emilio Vaccari (PULR). Aspects of this work were fundedby grants from the Brazilian agencies CNPq, CAPES, andFAPESP (to M.C.L. and J.S.B.), Samuel Welles Fund–UCBerkeley (to M.D.E.), Agencia Nacional de PromocionCientıfica y Tecnica (to F.E.N.), and Santander Bank (toJ.S.B.). Comments by Mike Benton and an anonymousreviewer were much appreciated. This is contribution No.15 of Laboratorio de Paleontologia, FFCLRP-USP.

VIII. APPENDIX 1. INSTITUTIONALABBREVIATIONS

BMNH, Natural History Museum, London, UK; GPIT,Institut fur Geologie und Palaontologie, Tubingen, Ger-many; MB, Humboldt Museum fur Naturkunde, Berlin,Germany; MCP, Museu de Ciencias e Tecnologia, PUCRS,Porto Alegre, Brazil; PULR, Museo de Ciencias Naturales,Universidad Nacional de La Rioja, La Rioja, Argentina;PVL, Fundacion ‘‘Miguel Lillo’’, San Miguel de Tucuman,Argentina; PVSJ, Museo de Ciencias Naturales, UniversidadNacional de San Juan, San Juan Argentina; QVM, NationalMuseum of Natural History, Harare, Zimbabwe; SMNS,Staatliches Museum fur Naturkunde, Stuttgart, Germany;UCMP, University of California Museum of Paleontology,Berkeley, California, USA; ZPAL, Institute of Paleobiologyof the Polish Academy of Science, Warsaw, Poland.

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